I've now tried to do it through the command line, and also failed:

[root@satellite1 ~]# hammer -u Admin -p <password> repository-set available-repositories --product-id 103 --id 9706 --organization <org>
-----------------------------------------------------|--------
NAME                                                 | ENABLED
-----------------------------------------------------|--------
Red Hat Satellite Tools 6.7 for RHEL 8 x86_64 (RPMs) | no
-----------------------------------------------------|--------
[root@satellite1 ~]# hammer -u Admin -p <password> repository-set enable --product-id 103 --id 9706 --basearch x86_64
Could not enable repository:
  400 Bad Request
[root@satellite1 ~]# hammer -u Admin -p <password> repository-set disable --product-id 103 --id 9706 --basearch x86_64
Could not disable repository:
  Repository not found
[root@satellite1 ~]# 

I just want to get rid of it from the right hand side of Content -> Red Hat Repositories (and therefore from sync plans, etc.).  I'm not using it in any Content Views.

8980  | yum       | Red Hat Satellite Tools 6.6 for RHEL 8 x86_64 (Debug RPMs)
8979  | yum       | Red Hat Satellite Tools 6.6 for RHEL 8 x86_64 (RPMs)
8981  | yum       | Red Hat Satellite Tools 6.6 for RHEL 8 x86_64 (Source RPMs)
9707  | yum       | Red Hat Satellite Tools 6.7 for RHEL 8 x86_64 (Debug RPMs)
9706  | yum       | Red Hat Satellite Tools 6.7 for RHEL 8 x86_64 (RPMs)
9708  | yum       | Red Hat Satellite Tools 6.7 for RHEL 8 x86_64 (Source RPMs)
10314 | yum       | Red Hat Satellite Tools 6.8 for RHEL 8 x86_64 (Debug RPMs)
10313 | yum       | Red Hat Satellite Tools 6.8 for RHEL 8 x86_64 (RPMs)
10315 | yum       | Red Hat Satellite Tools 6.8 for RHEL 8 x86_64 (Source RPMs)

I can do the following:

[root@satellite1 ~]# hammer -u Admin -p <password> repository-set disable --product-id 103 --id 8979
Could not disable repository:
  Repository not found
[root@satellite1 ~]# hammer -u Admin -p <password> repository-set enable --product-id 103 --id 8979
Repository enabled.
[root@satellite1 ~]# hammer -u Admin -p <password> repository-set disable --product-id 103 --id 8979
Repository disabled.

So it appears Satellite's record for 9706 is partially deleted and for some reason I can't reenable it to disable it again due to a bad request.

Please, how do I fix this?

For reference, I was able to solve this issue with the help of Red Hat Support.  The sequence was:

# su - postgres -c "psql foreman -c 'select * from katello_repositories'" > katello_repos.txt
# hammer repository list > repo_list.txt

From looking at the output text files of those, we extracted the repo ids for the problem repos (note that I have two Organizations in my Satellite, and both had the RHEL8 Tools 6.7 repo 'stuck' enabled, hence two entries:

-----|----------------------------------------------------------------------------------|---------------------------------------------------------------|--------------|---------------------------------------------------------------------------------
ID   | NAME                                                                             | PRODUCT                                                       | CONTENT TYPE | URL                                                                             
-----|----------------------------------------------------------------------------------|---------------------------------------------------------------|--------------|---------------------------------------------------------------------------------
6677 | Red Hat Satellite Tools 6.7 for RHEL 8 x86_64 RPMs x86_64                        | Red Hat Enterprise Linux for x86_64                           | yum          | https://cdn.redhat.com/content/dist/layered/rhel8/x86_64/sat-tools/6.7/os       
6599 | Red Hat Satellite Tools 6.7 for RHEL 8 x86_64 RPMs x86_64                        | Red Hat Enterprise Linux for x86_64                           | yum          | https://cdn.redhat.com/content/dist/layered/rhel8/x86_64/sat-tools/6.7/os

A quick check to see if they were 'valid':

# foreman-rake console
irb(main):001:0> repo = Katello::Repository.find(6677)
[output removed]
irb(main):002:0> repo = Katello::Repository.find(6599)
[output removed]

Then I could use this set of commands to delete it:

irb(main):003:0> repo = Katello::Repository.find(6677)
irb(main):004:0> repo.destroy!
=> nil
irb(main):005:0> repo = Katello::Repository.find(6599)
irb(main):006:0> repo.destroy!
=> nil
irb(main):007:0> exit

The web GUI confirmed in both "Content -> Red Hat Repositories" (Enabled Repositories column) and "Content -> Sync Plans" (drill down into the plan and look for the repo under "Products -> Red Hat Enterprise Linux for x86_64 -> Repositories") that the problem repo was no longer visible for either Organization's Sync Plan.

Upon review of our valid but aging backlog the Satellite Team has concluded that this Bugzilla does not meet the criteria for a resolution in the near term, and are planning to close in a month. This message may be a repeat of a previous update and the bug is again being considered to be closed. If you have any concerns about this, please contact your Red Hat Account team.  Thank you.

Thank you for your interest in Red Hat Satellite. We have evaluated this request, and while we recognize that it is a valid request, we do not expect this to be implemented in the product in the foreseeable future. This is due to other priorities for the product, and not a reflection on the request itself. We are therefore closing this out as WONTFIX. If you have any concerns about this feel free to contact your Red Hat Account Team. Thank you.