Is it possible to deactivate the "@" function on the activity tab? Currently all users are able to mention any other user, which is not feasible.

I’m not sure. i believe it should respect the permission system but I’m not sure either (so if a user doesn’t have access to another user they shouldn’t be able to mention them either) This has been an issue since the release of that feature, never addressed.

We've tried this and it did not work - which makes the whole activity tab more or less useless to be honest... Being able to handle this via permissions would be a huge improvement!

Yeah, that was what I was afraid of AFAIK nobody has been able to use this feature unless the whole app is being used for internal use only.

That's too bad - are there any plans to apply permission to the acitivty tab in the near future?

There are no immediate plans, but I will pass this on to our product team!