How about "I'm headed in a different direction than the kind of stuff you play... but I'll keep you in mind if something ever changes (i.e. you start dating Britney Spears' sister)"

Or "I enjoyed your stuff, but there's a bunch of others auditioning that I need to get to first. I'll let you know if we can work something out"

I dunno... it's not really that hard to be polite while turning someone down. Happens every day.