BuddyPress.org

Check buddypress/bp-activity/bp-activity-filters.php for a list of what it’s being run through already.

bp_get_activity_content

…is the name of the filter you will want to use, just like BuddyPress already does.

You could make your own custom kses for it, or if you’d rather try strip_tags give something like this a shot?

http://pastebin.com/m9cb9d10

The problem with that code (as it is) is that the activity stream contains the href’s and spans for the links to users and the time, so I don’t think strip_tags will really work. You’ll probably have to put your own kses in there honestly.