Someone I know creeping me out, help?

While rude to ignore him, you made it pretty clear before that you didn't want to spend time with him and he didn't get the hint. It would be much easier for you to just completely blank him in future. It seems cruel, but based on what you've said, if you greet him out of politeness he may take it as an encouraging sign and then you could end up back at square one. Don't feel too guilty, the guy is pitiable, but it's not your fault.