Start reporting on those messages. If you are on Android, use India Against Spam. If iphone, you'll have to report manually on the operator's website. for example - airtel.in/dnd

It's a pain in the ass but I kept on reporting for a month after activating DND and the calls and messages dropped. I've even received calls from people begging me to take my complaints back or otherwise they'll loose their jobs. I agreed twice but holy hell those exact same companies spammed me again. I stopped being a good samaritan and just told them to sort it out with their companies since it's the companies fault. Now spam has reduced to a message or two a month.

From what I understand there are old mobile numbers list being passed around from before DND was available and that's why you need to start reporting the spam messages. Word quickly spreads between the companies to remove your number from the lists.

I have reported them via India Against Spam but the responses from Airtel were, roughly, 'thanks but this is not spam' so I stopped reporting. Complaining on Twitter helped with some, but not all, spammers.