Capping light fixture box with two white and two black wires
If this is in the middle of the run to the other lights being controlled by the switch (i.e. capping each wire individually means that some lights won't work, and there's no reason the electrician would have installed a temporary porcelain fixture if a simple capping would have done the trick) then:

Turn off power at the breaker.
Remove temporary porcelain fixture.
Using a yellow or red wire nut* connect the two black wires.
Using another wire nut, connect the two white wires.
Double check both nuts are tight by holding the nut and trying to pull each wire out.
Push the wires into the box.
Attach a round, blank cover plate.
Turn on power at the breaker.

*Check the package labeling to ensure you're using nuts of the correct size for the wiring you're using. My box of orange says it will not support two #12 or two #14 wires, only 1 of each with something #16 or smaller. My box of yellows says it will support two #14 or two #12, as does my box of reds.
Of course, it strikes me as very unusual that the electrician wouldn't have done this in the first place. Surely he would have had a blank cover plate on the truck just as readily as having a porcelain. OTOH, he may have made more profit by charging you for the &quot;temporary&quot; porcelain than he would have for just putting a cover plate on it in the first place. :/