Joe-jobbed...What are my options?

[Steve]

I've recently had my domain targeted by a variety of offensive spammers 
pushing legally dubious stuff who have chosen my domain as the sent-from 
and/or reply to address in forged email.

My simple question (which I admit is a bit spamassassin off-topic) is 
"what can I do about it?"

Sorry if this is really simple... any advice would be useful.

Thanks,

Steve