Ceiling Fan Toggle Switch No Longer Used - Can I do anything else with it?
You can get a double blank plate with one toggle and the other part covered, like:



What you need to look for is 1 toggle 1 blank combination wall plate.

There are varieties available for regular toggle, Decora, etc.

You will need to cap any unused wires, from the switch you remove, with wire nuts, plus a little electrical tape to make sure they don't come loose (but the wire nut provides the actual insulation needed).