Not op, but I usually use the web form or an email address provided by the company. An email is great for having a "paper trail." Realistically, though, they either will comply or else there's not much you can do.

I include a snippet like this one, which is for paper mail:

I'm writing to request that you please remove me from your postal mailings list. I no longer wish to receive your mailings. Please, also, do not rent or sell my name or address to other organizations.