How can I temporarily stop a bathroom water leak?
HomeDepot carries a brand called "SharkBite" that are simple, push-to-connect fittings. You can buy a cap from them for cheap. It requires an equally inexpensive plastic tool to remove. That, or something like it, should get the job done.

The cap: http://www.homedepot.com/p/SharkBite-1-2-in-Brass-Push-Fit-End-Stop-U514LFA/202270531

The removal tool: http://www.homedepot.com/p/SharkBite-1-2-in-Disconnect-Clip-U710A/100638139