Security breach/bots

After 12 months of using JooBB we now are getting malicious posts added to our FORUM, which actually lives in a secure pw protected site.

We are seeing posts for:

gibbrish, like "kj234lkh089sdfhj/isodfi9"
or "Get ringtones" or "Buy Vgra"

These seem to be posted under a "guest" account.

How do we prevent these unwanted posts from happening please?

THANKS!