You don't say how this person is doing this. In fact I can't see how it is possible for anyone (BT Employee or not) to control a Set Top Box remotely unless they are using a remote control pointed at it.
However if you are sure that he/she is doing it then it is an offence to interfere with Broadcasting and the person can be reported to Ofcom. www.ofcom.org.uk.
t
May 2008