Turning off “new topic” and reply buttons

I want to be able to disable the “New Topic” and “Reply” buttons based on some custom conditions (like whether a user belongs to a user group). I checked the code but I don’t see any functionality (filters/actions) that would allow that. I could do it with CSS, but I’d rather not do that since its a relatively simple matter for someone to edit the CSS.

any ideas if a filter could be added to control menu output?