Whozurdaddy gave the best advice for your problem, just to add a few notes though-
1. If he is gaining access through teamviewer I doubt it was your ip address in which he is using, teamviewer generates a its own access code and password for people to gain access. (this is not to say he didn't obtain an ip address while he was accessing it) but to stop teamviewer access just change (or create) a user-defined passcode. Also check the box that automatically records all incoming connections and monitor its save folder.

2. If he is gaining access with an ip address, log-in to your router and change your ip renewal policy to 1 day which should be the default. Ensure that your router config setting is set to a DHCP standard, which will allow for your router to change your ip every day, once again this should be your routers default.

3. Next open your task manager, select the services tag, click the services button in the bottom right hand corner, in the window that pops up filter the srvices by name and scroll down to the ones that say Remote there should be like 7 or 8 disable all of them.

This will stop him if he has basic knowledge, if he is comp savvy and a possiblke background in "hacking" lol you will need a much more in-depth approach

Fl Studios is decent but Pro Tools would be a better choice for serious users. VirtualDJ and DUBTurbo have a MUCH easier learning curve for those just starting out but if your looking to go professional, want EVERYTHING POSSIBLE at your fingertips when it comes to mixing music, or just have a good chunk of money sitting around, Ableton is the way to go hands down. I've enjoyed using Mixcraft as well!