There are groups that already do these types of things.

Oh nice! Would you happen to have a few names to guide a Google query?

ideally we'd stop the scams at the source, rather than flooding the internet's infrastructure with even more pointless traffic

but i'd still enjoy seeing a project that automates the wasting of scammers' time

even something as simple as piping their emails and responses to/from cleverbot could be moderately effective

People are notoriously bad at spotting bots. Just a bunch of canned messages can draw people into surprisingly lengthy conversations when they are desperate to get something out of you... The biggest problem would probably be to vary the exchanges enough to prevent them from learning to pick them out after a few exchanges.