How to Tell Colleges You're Not Going to Their School

So, 2/3 colleges I plan on not going to gave me an envelope and a card to tell them that I won’t be attending their school. But for the last one, they sent me nothing. How do I tell them I’m not attending their school? Do I have to call or send an email?

Or do I just let it be and not say anything?

Perhaps they have an electronic option on their admitted students page. Failing that, I think email is OK; anything in writing is preferable to a phone call. Doing nothing is just rude IMO; they will not break down in tears because you are not going, but you never want to burn any bridges.