So, for the moment, i haven't found what the problem is...
Have you try to deactivate all your extensions except the shoutbox, made a test, then re-activate them one by one doing a test at each plugin re-activation ?
Keep me inform. From my side i'll try to explore more deeply what could be the problem.
Sorry, but for the moment i have no solution :(.
PS. Does the number of spams increase ?