I was having some trouble with the community events feature for blocking certain roles from the admin. The function does a simple if (is_admin()) but the problem is that this may block frontend ajax functions that call the wp-admin/admin-ajax.php file, which happened to me. E.g. I have an ajax call that checks for Event ‘overlap’ to prevent double bookings, but I blocked authors from backend, so the function did not work for author-users.
Something like this may work better: http://pastie.org/5498781
function is in /lib/tribe-community-events.class.php line 2748. Love the plugins! Hope this helps others in the community.