I had the same issue where I also seem to have been subscribed without my knowledge. I am on post-paid, and by the time I notice, I had spend around KES 600 for 2 months. I managed to unsubsribe for another KES 30.
People on post-paid need to check their bills at http://selfcare.safaricom.co.ke and see if they are being charged by these scammers.
And once you are subscribed, it is KES 30 every day.