To follow the National Electric Code and to be safe, the fixture boxes cannot be plastered over. As long as live wires remain in the box it must remain accessible (even if they are switched off). This means that you may install an UL approved cover but you cannot seal it from access. If the unused wires are removed altogether, the holes may be patched over. However, this may be harder than it appears and may affect other outlets or lights elsewhere in the house. The wires and boxes could be relocated to the attic (which is generally considered by the NEC to be accessible) but may require someone with a good working knowledge of electrical systems. Hope this helps.

Regards

Thanks for the great info. I knew none of it. I will definitely not attempt to do the work myself. Thanks for the help!