I have OpenDNS setup for one of my kids. After you configure OpenDNS on their website, log into your router and:
1. Go to Parental Controls
2. Click on DNS Filtering
3. Slide the switch for filtering to ON.
4. Change Global to No Filtering.
5. In the Clients List, select the offender you wish to block.
6. In the Filtering List, select OpenDNS.
7. Now click the + (Add) button.
8. Click the Apply button.
9. Go test the "client" you just blocked.
I set this up for my daughters devices (Roku, laptop, etc).
FYI, in my setup, I use Cox cable for TV and internet services. We have DVR units; however, the way they are configured (by Cox) is that they are NOT connecting via my internet and therefore I have no parental control of them, outside their built-in tools.
Perhaps DN is different. If I am reading your question correctly, the DN equipment is what your "offenders" are using to record shows and watch Netflix, YouTube, etc. and jamming up your storage. Assuming it's the same device, the question is making sure the DN is connecting through your router and that you can select each device as a client in the instructions I posted above.
Just a few other thoughts:
1. Change the DN password, keep it secret and utilize any admin tools available via the DN interface.
2. Make the users aware they have X time to watch a show and delete them. After a few missed shows and some crying, they will get the jist of how it will work.
3. Remove their access to the DVR system altogether. Give them a dummy system so they can't record/view but still watch live TV. Probably save you a few bucks in the process.