you should usea 30 amp junction box or 30 amp conector blocks, (or crimps) not tape you then put a blanking plate over the socket, sorry but you can not just plaster over it, you never know you may need a socket there one day, easier to leave it.
As a thought, you could remove the cables from the wall and put a junction box on them and put it under the floor boards, then you can plaster over the wall
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