You could use a blanking plate - http://www.screwfix.com/p/lap-1-gan...tracking url&gclid=CM-qmqCRz9ECFRa3Gwod9NQISQ
If your lucky and just a one gang switch with is just 2 wires. You could disconnect at the light it feeds. Then prove dead and terminal block and plaster over. Maybe send a picture wait for what sparks say
If you simply disconnect at light fitting your still going to have a live feed at the switch Better to disconnect switch and pull cable out or at least disconnect from where it's fed - junction box / ceiling rose, etc Then can happily fill and plaster cut out
I was thinking along the terms of the permanent live supply cable from the consumer board goes to the fitting first. And surely it then disconnected the live and the permant live are then dead. Pull out cable what ever then. Maybe I'm missising something? But you do say ceiling rose after which is what I said. Guessing it's depending how it's wired Dave happy to be corrected