Is there a way to prevent myself from changing dns settings?

I think you should do the following... It won't prevent you fully to change them but it will give a decent amount of time to stop the urges...
Change the password to one you surely won't remember(random characters and long), both on opendns and your router.
Write that password dosn on a paper and save the paper somewhere on your house that's not easily accessible. Don't save the password on the chrome password manager or other types of password managers. Just on a paper. Hide that paper well, so that if you actually need the password you can find it, but as it's not that accessible, you might havd a few minutes to reconsider your urges.
Also on that paper write something like "go on nofap and read at least 3 succees stories". Anyways, this won't prevent you 100%, as you need to put some from you part, but it can save you some relapses and even all relapses if you also add some resistance and perseverance from your part.

By the way, good luck and be strong