A postal address would be better. You can send them a letter saying that you do not want any more nuisance phone calls and any future correspondence should be made by letter. If you get a "proof of posting" (free from any post office) for your letter (or recorded delivery if you want to pay extra), then the company will be breaking quite a few rules if they keep phoning you. You can report nuisance calls to your network provider if they continue.
It would be best if you keep communications as polite as possible. Even if you feel like being rude, you need to remember that it isn't necessary.