Since upgrading to WP3.0 and BuddyPress 1.2.5 I found that marking users as SPAM or Deleting them would result in the following error:

Fatal error: Using $this when not in object context in /home/mysite/public_html/wp-content/plugins/buddypress/bp-groups/bp-groups-classes.php on line 1053

I found the solution here:
http://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/fatal-error-when-trying-to-delete-user/#post-61229 from Michael Sumner

Open the file mentioned, and replace this:

$group_ids = $this->get_group_ids( $user_id );

with this:

$group_ids = BP_Groups_Member::get_group_ids( $user_id );

And right below that, replace this:

foreach ( $group_ids->groups as $group_id ) {

with this:

foreach ( $group_ids['groups'] as $group_id ) {

Already fixed for 1.2.6 http://trac.buddypress.org/ticket/2479