A bit late, but here's my tuppence worth...
The BBC do not collect the licence fee, the TV licence people are completely separate (it's a hypothecated tax rather than a fuction of the BBC).
The licence is for equipment capable of receiving a live broadcast. The iPlayer is therefore largely exepmt as there isn't all that much live broadcast stuff available - but if you don't have a licence you shouldn't watch it. For the moment if you don't have a telly you'll be fine without a licence.
I'd suggest giving them your details and letting them in for a look round, it'll mean the letters should reduce to about once a year.
If they keep on at you reply and say that it's harrassment and you will contact the police if it doesn't stop. This will work better if you have been compliant (let them in for a look, given your details).
Remember - they have no right to enter your house. The only people who can enter your house whenever they want are Customs & Excise. The TV Licence people can't demand entry. If they turn up with the police even they don't have a right to enter your house without a warrant, but I suggest you don't let it get this far.
In summary, you will be punished for not owning a telly but if you comply now it should become minimal fuss.