Someone keeps using my email address. What to do?

This is not very uncommon. Unfortunately you have no single solution, unless the individual is in the same locale as you (in which case you may be able to take legal action if he is causing you to incur costs)

Generally, there is no technical solution, as this is a human problem. They could be doing it deliberately, as Pacerier suggests, or they may just think that they have the email address right.

Online services typically just let people signup with whatever address they give. The better ones require email validation - so as long as you don't validate the email, those ones shouldn't cause you any problems.

In summary - you can try and educate the guy, but basically you'll just need to ignore the issue.

This doesn't solve the root problem, but might be a useful workaround. You can start giving your address out as [first name][2 digit number][+][last name]@gmail.com. Gmail ignores everything after the + and you can filter for emails which don't include your last name. (It would probably be worth going through the filtered emails every ~week to catch emails from people who you haven't told to use the new address yet.)