I am able to block unwanted SMS from numbers by blocking them using the smssecure application.
I started using smssecure so that my text messages would be encrypted (no data connection needed),
it saved my bacon against stagefright stuff, and now I can easy block unwanted numbers.
You can get it from the google play store,
but also from smssecure.org
or via de f-droid open source repository, if you have trust issues with google.
Select in smssecure the conversation of the person you want to block,
got to conversation settings via the menu, select block, confirm and your are done.