How should I eliminate an unused switch in a two gang box?
Hold on. You can't just eliminate lights

You may notice that in almost any room you go into, anywhere, you have an intuition as to where to reach for the light switch.  The light switch is in an expected location.  That's not by accident.  That's established in the Building Codes.   It is mandatory.  

It also comes up in NEC Section 210.70(A)(1), which provides rules on dwelling unit lights and switches. source

So as you eliminate light switches, you must make sure you don't eliminate the legally required ones. 

"But we're not moles, of course there will be light, you'll just have to know the secret way to turn it on".  Nope. Nope. Secret ways are not allowed.  They may work for you, but won't work for guests, particularly First Responders. 

If in doubt, post a room layout drawing here showing lights and switches. 

How to do it

In normal 1-way switches, they have 2 switch terminals.  That is all.  

If there are 2+ wires going to the same terminal, I recommend doing something to indicate to the next person how this was wired; e.g. Mark the solo with red tape (if it's the switched-hot), or bundling the group with tape.  

Then, simply join all wires with a wire nut.   

Then, fit appropriate cover plates, as discussed elsewhere.  They also make little "switch plugs" that bolt up like a normal switch, but have no wires and simply a rectangular plug that fills the hole.  

This allows  "the next person" to easily reactivate that feature if they find it useful, or if you're selling and your inspector catches a missing mandatory switch.