BuddyPress.org

The Widget renders from bp-core-widgets.php (look for BP_Core_Recently_Active_Widget).

It calls bp_member_avatar that renders out the avatars using the filter bp_get_member_avatar (see bp-core-templatetags.php).

I assume that it would be possible to alter the filter accordingly so that it only shows files other than the mysterman.

of course you would probably have to think about how to keep the amount of members the same since cutting 9 out of 10 off the filter and have the widget show 15 users it would end up showing only 1 user .