Facebook Messenger

Since this is not an Apple app, you will want to check with Meta on any solutions. You can also give them feedback as they are the only ones that can update the app to improve user experience. It is likely that they include the Suggested Communities in an attempt to increase engagement so you use their app longer and may be unwilling to remove it.

You would need to contact Facebook regarding any changes to their service. But, as Facebook wants you to be involved in every community possible, they would have little interest in making such changes.