1. Go to the Dashboard of the domain in question
2. Scroll down to the keyword section. Check the keyword(s) you wish view the ranking history of. The box at the top of the keyword section can be marked to check all keywords.
3. Once you have checked the keyword(s) you wish to stop to track, scroll back up to the top of the keyword section. Click the "Delete"-button
4. Confirm the deletion if you are certain you have chosen the correct keyword(s)