|Do you still have contact with him? You might warn him that you are going to approach the Police about this matter if he continues as it is a criminal offence in most countries.|
The problem is more likely not that he has access to your computer but that he knows your logon names and passwords for various sites. Change the passwords on all such accounts.
The only way you could be 100% sure would be to wipe your PC and reinstall the OS.
Oh, and if what you say is true you might want to take legal action against the "idiot" to recover your $300.