How do I remove pairs from My Watchlist?

How do I remove pairs from My Watchlist?

Highlight the pair in My Watchlist by clicking on the very left edge of its row.   Go to the upper toolbar and click on "Delete Pair".  Job done.

If you wish to delete a Group, you first need to ensure that all pairs in that Group have been deleted from My Watchlist, then go to System Preferences/Securities Groups, highlight the Group, then click on "Delete".