To take a few steps back up the thread to the part about turning down future dates, the same goes for turning down shitty job offers.
"[Thank you for your interest]/[Thank you for thinking of me for this role], however I'm afraid [I have prior commitments]/[I'm not a good fit] at this time. I wish you the best of luck [finding the right fit for your company]/[with your startup endeavor]"
Delete or substitute more relevant lines as appropriate, but it's a polite way of saying "no" without feeling like you need to overexplain.
Increasingly, people -- millennials and younger, rarely anyone older -- will not get the hint about this and get pushy, fishing for extra information to let them work around the Polite No, which in previous generations would have been incredibly rude. It's still incredibly rude, at which point the polite response is, "Thank you, but I gave you my final answer. Best of luck!" no matter how many times they come back. Become a broken record until they go away.