I wonder how hard this would be to block at the MMSC end? Although we all know how much networks like to get off their arse and do something useful... They love acting as dumb pipes, but only when it suits them!
Oh well, given I haven't received an MMS in over a year, I just mangled the details in the APN... That should keep things safe... At least until an update arrives, which I expect won't be long on my Nexus.
I fear for the security of OEM devices though.