How can I keep a hacker out?

You're going to have to figure out where he's getting the information from.

If it's as simple as knowing the answers to your password recovery questions, for example, then just change them.
In fact change any & all passwords he may know, or know the recovery answers for, & make sure his email address isn't listed as a secondary or recovery address anywhere.

There is the possibility, however remote, that he may also have put key logger or remote access software on your computer. That's going to need a good set of antivirus tools to locate & eliminate.
Remote access could include things like TeamViewer. If he set up password-free access from his side, then you'd need to check & change its security settings to revoke that privilege.