1) why didnt u mention that they get banned first time they visit!
2) what 4images version?
are u sure that sessions on your 4images are working fine?
this mod is using 4images sessions to "cache" the results (to avoid extra server load).
u can try disable "caching" feature by replacing $force = false; with $force = true; in includes/functions.php (require Ban v1.5 or above)