To remove Printopia 3, drag the Printopia app into the trash then empty it. This will remove all components of Printopia 3 from your machine.
To remove Printopia 2, open System Preferences, and on the main screen where you see the icons for each panel, find the Printopia icon near the bottom. Then, right-click (or control-click) on the Printopia icon. In the menu that appears, choose the "Remove Printopia" option.