Registered users hacked, posting garbled text

[Jouster]

I have an interesting problem - old, longtime members of my forum seem to have been hacked and are posting strange strings of text. This is only new threads (never replies to existing ones), and the users can still use their accounts. What can I do about this? I followed the steps to make my forum more secure, but it is still happening (though the akismet anti-spam is sending it to moderation queue, at least). Has anyone encountered this?

My forum is at http://aspecialthing.com/forum btw, though you won't be able to see the threads since they are being moderated.

[Steve Machol]

The first thing they should do is change their passwords.

[yingzhou]

I have this too, I think its about the 3.7.4 PL1 patch.