Auto complete is a good option for finishing a PR and automatically merging the changes. But, an option to disable this button for some projects/repos would be nice. Even better if we can disable this only for particular teams. We have a case where completing a merge starts a release and if a Pull Request is approved accidentally, we've pushed bad code to master.
As far as I know, it is not possible we have had this issue in my previous team. You have two options:
Check Microsoft doc for policy.
If you already use policy, you could consider using the git-flow strategy, so you always merge to develop branch instead of the main, but that will slow the release process.
Finally, you can suggest new features for Azure DevOps, read here.
You can also read about the latest updates of Azure DevOps.