Prevent Spying

I am looking for advise on solutions to prevent spying on my home network. I know my former roomate was spying, as the person was able to show me account details including password that was recenty created.

What are my options to block this. I am guessing something like MSPYor PlexiSpy is being used. I just did a complete reformat of my PC (with brand new drive) so the system itself should be clean. Would a VPN service be the best option? Any ideas would be appreciated.

Thanks