For the common Hangouts use case:
Settings -> SMS -> Auto Retrieve MMS -> Uncheck
edit: Ah it's in the actual hangouts app that you have to go to settings. Also if those settings are greyed out then you might not have Hangouts handling your SMS messages, so run the Messaging app instead and go to its settings to disable auto open of MMS.
Edit: Messaging (default app) doesn't have this setting, but Messenger does. I tried Hangouts for awhile but didn't like it.
now if you want to make calls from your gvoice number, you must use hangouts.
funny how they think forcing newer apps is the solution to the g+fiasco
Contrary to the NPR article, somebody further down this page said this hack may affect Messenger users too. So I'm sticking with Hangouts!