Weird spam message overlay appears on my iPhone's screen (FLASH SMS)
Chances are your phone number was used by someone (else, or had belonged to someone before it got cycled to you) and they had subscribed to those services. If your phone bill is clean meaning there are no odd fees showing up out of the blue then I doubt it is an app.

My personal experience. I once installed an App on my iPhone and like a dumb idiot in a rush to get to work I loaded and accepted the usage terms (why would I read 45 pages).... well shortly there after I was getting billed an extra 20 Euros a month. Had to change my contract with my provider.

Another story, I was recently overseas and needed a prepaid SIM for my phone. I put it in and everything seemed fine until I was getting random SMS with jokes, weather, and sports that I didn't need nor had personally subscribed too. But the person who had my phone number before me had subscribed.

Best thing to do is contact your provider in any case and see if they can block the attempts on their side so that you don't get harassed anymore. Sorry to hear of your woes.