Add-on [request] move post if "Default" prefix is not selected

I have been noticing a trend with current spam that submits a thread in a forum without using the default prefix, Instead no prefix is submitted and 100% of the time this happens it is spam, I have removed the prefix drop down with css and humans abide but bots do not.

This is one particular forum that is open to guests and this mod or code edit should keep us satisfied until the bots start submitting XF default prefixes.

I would like a mod or code edit to automatically move such threads to a "Trash" forum.
Shifting this into the "Paid" category as the bots now have me in their sites.

I know many spam options are publicly available but I am unaware of one that is for guest posting, Thus the request for this particular mod.

Anyone interested ?
(I don't need admin options - Hard coded to trash forum ID is ok)