I think add warning above thread in digital goods with big/bold font should work well even though it's annoying.
This is also a possibility, though may be overkill (and I'm sure many would complain about it as they do with the newbie red-font PM warning).
if admin ban posting url of such website then it would help as scammers won't able to post autobuy links like posting links of some websites are already banned so adding autobuy sites to that list would help.
This has also been suggested but likely only preventing Newbies to post them (which would stop those url spambots as well). Persistent scammers would just farm accounts to get around this though and some seem to already have an arsenal of farmed accounts that instantly reactivate once their previous account gets banned.
