I might try subterfuge along the lines of 'OK, this isn't my phone, it's my boss's and I'm just answering it while she's on another call. Could I have your name and telephone number so she can call you back?' or get a colleague to do similar.

I could always get a whistle out and blow it down the phone line.

Assuming I can get a name out of them, I can create merry havoc using the DPA - the Information Commissioner doesn't have very many teeth, but I can enforce my rights in the County Court. If it reaches the stage of daily calls, then the cost/benefit analysis of starting proceedings will start to look favourable.

As a matter of principle I answer all telephone calls on the basis that I shouldn't have to deprive genuine callers of the ability to contact me simply in order to avoid companies trying to sell me things.