You can specify a list of forumids as a parameter to external.php, like
Code:
/forum/external.php?type=rss2&forumids=1,2,3
but unfortunately there is no parameter to exclude a list (and no parameters for individual threads). Of course this doesn't do any good if you're trying to keep people from getting feeds to those forums (because they don't have to include the parameter). What you could do is create a plugin using hook external_start and set your own list of forumids.