If you cannot disable strict mode (you must be root to edit the my.cnf file) you might be able to change the column type from a TEXT to a LONGTEXT to support absurdly large posts... You'd want to do both POST_BODY and POST_DEFAULT_BODY, after making sure to make a backup before making any unsupported changes.
Please note that the vendor cannot support any changes to the stock behavior of UBB.threads.
