Cant be simpler, simply contact one or both of the two addresses below:

Unaddressed Mail- Opt-Outs, Royal Mail, Kingsmead House, Oxpens Road, Oxford, OX1 1RX

Addressed Mail - Mail Preference Service (MPS), Freepost 29, LON 20771, London, W1E 0ZT or call 0845 703 4599

Done and done