Head to the Position Tracking report, by clicking on Tracked Keywords in your project. 

In the list of tracked keywords, you can either select all (by checking the first box) or select specific keywords you want to remove.

Once you have selected at least one keyword, the “Delete” button will appear next to the “Add Keywords” button

Important ⚠: if you are tracking more than one location, you will have to do the same process for each specific location.