How can I safely remove an outlet from service?
There should be no problem with opening the box, removing the outlet fixture and then putting wire nuts on the ends of the wires. It may be necessary to straighten the wire ends and trimming slightly if the bare wire ends are too long to be nicely covered by the wire nut. Some folks go the extra step of wrapping the wire nut and the end of the wire with electrical tape. 

After the wires are appropriately prepared fold them down into the electrical box and cover the box with an appropriately sized cover plate. The key here is to keep the electrical box cover accessible by not trying to cover it over with permanent building materials and finishing.