From Sandeep’s blog:

Public Service Announcement

Are you an Airtel user bombarded with SMSs and automated voice calls every day? Then all you need to do is send “BLOCK” to 222 from your mobile. I don’t recall Airtel ever having publicized this for whatever reasons, but I got pissed off with their recent ads and called them up today to get this answer.

Update:
Apparently, it takes 48 hours to de-list me from their services. Shit, I can’t wait for so long!

Anyone know how to do this for other operators?