What should I do with a double gang box if I no longer need two separate switches?
I'm surprised that the switch controls the entire outlet -- generally in that setup (in North America at least), the switch only controls one plug, and the other plug is constant power. In that case of course, the answer is to use a power bar. It may even be worth checking in the plug to see if there is constant power available. You may be able to fix the wiring to work as I've described. 

If you can't fix the problem at the plug, then remove the switch, connect the wires together with a wire nut, and then install a duplex faceplate with a blank. The only way I know of to do this is to use the "decora" style blanks, and then either a duplex decora plate, or a combination toggle/decora plate.