1. Turn off the electricity on that circuit, and check that it's off, and make sure that nobody else in the house will turn it back on whilst you are working on it.
2. If it's a spur, disconnect the socket and terminate the wires.
3. If it's on the ring, disconnect the socket and reconnect the wires to each other.
Termination or reconnection will depend on what access you have to the cables. If you've got floorboards that you can lift, then you can pull the wires out of the backbox and either terminate or reconnect them using a junction box which you leave under the floorboards. Actually, if you do have under floor access, and it is on a spur, and you can find where it's spurred from, you do have the option of removing the spur cable altogether.
If you can't get access to the cables except inside the backbox then you'll have to terminate or join them in there - again use a proper junction box if it will fit, rather than a strip type of connector - they are safer. If for any reason you do have to use a strip you should put it inside a proper box (they look like this:
http://www.tlc-direct.co.uk/Images/Products/size_3/CHOCBOXB.JPG ), but you should be able to get a proper junction box inside the backbox (you might find that a rectangular one like this
http://www.tlc-direct.co.uk/Images/Products/size_3/MK1130.JPG fits in easier).
[BTW - I have no association with TLC - their prices are good and their website well designed, but other suppliers are available].
DO NOT BE TEMPTED TO USE INSULATION TAPE - joining wires by twisting them and covering them with tape is extremely unsafe. Terminating by covering with tape is OK if you are careful, and if it's only temporary, but over time the tape will lose adhesion, fall off and create a hazard.
4. Check that you've not made any mistakes. Check that all the screws are tight. If you've disturbed wires under the floor or in a stud wall, check that you haven't left them where a nail might penetrate them.
5. Depending on how you've terminated or reconnected, you can now either put a blanking plate in place of the socket, or (later) fill in the hole and paint/paper/whatever over it.
6. Turn on the electricity.
Mike
