On the form side it's relatively easy in Safari or Chrome to just remove the maxlength="50" via the Web Inspector, so an automated tool could just ignore / remove it as well. To stop it from occurring the php code would need to reject lengthy subjects before adding them to the database instead of just relying on the user manipulatable html front end.
The PHP could validate the subject string (or, alternately, the database could store the subject in a varchar(50) field), but neither of those checks is done in UBB; the UBB stores the subject in a text cell of the database and relies on the browser to limit the length.
A bit silly, really, but there it is.