I don't know of any such exploit but you may want to consider upgrading to 6.7.2 and disabling UBB.Friend and allowing users the ability to mail users (i believe this was added in 6.6); additionally you may think about disabling Private Messages as users may be getting spammed through that (which is likely what your host is whining about).
Ask your host for more information, there isn't any part of the UBB which would allow spam other than the "allow users to email through the ubb" option.
