3-Toggle to 2-Toggle
Power off obviously. You can just patiently twist and tug the wire out of the backstab, and then put a wire-nut over it, and tape the wire nut so it stays. (Wire nuts are supposed to stay on splices without help, and shouldn't be taped then; however when capping a single wire they don't work properly and need help.) 

Pulling a wire out of a backstab destroys its spring strength;  (even when using the release tab). The backstab cannot be reused.  Throw the switch in the trash.  Unless (like most of us) you are a member of the "never use backstabs, ever" club, in which case the switch is fine; put it back in stock. (backstabs are unreliable and create maintenance headaches later, which is why we shun them). 

They make blanking plates that bolt to the same yoke screws as the switch did, and neatly fill that hole in the switch plate cover.  They also make switch-switch-blank switch plate covers, though they are rare, which is why I mentioned the first thing.