Lock down 'receive text messages' setting
If you don't want to receive text messages on your actual phone number and only receive them via your Google Voice number, just call your carrier and tell them to disable text messaging. 

I used to do that, back when I had a plan that charged for messages. Just tell them to disable incoming and outgoing text messages, and you won't have to worry about any future charges.

The Google Voice app sends and receives text messages as standard data, and it doesn't use the carrier's texting abilities. So disabling texting through your carrier won't inhibit your ability to use Google Voice for texting.