Hi, t_l_m
Good day!
Thank you for posting to Microsoft Community. We are happy to support you.
According to your description, the issue of your concern that you could not disable an add-in in Outlook.
Since you're using the third-party application, for the given issue that we're not much aware about it.
At this moment, we're afraid that we could not give you an appropriate suggestion to you.
Nevertheless, there's a dedicated expert team from Microsoft - office-outlook-itpro (Clickable)who dedicating into Outlook relates to third-party apps with more resources and rich experience.
We believe they would conduct in-depth research on the problem you encountered and provide you with more professional assistance.
Thanks for your precious time and your understanding would be highly appreciated.
Sincerely,
Alex | Microsoft Community Moderator.