Disable the merge button in PRs

I want to have the option to completely disable the green merge button for PRs in the curl project since it basically is always wrong for our project. I don’t think the UI can be improved enough to give us the exact control of the commit message(s), format and metadata (like fixing the author field) that we want and that’s fine, but we don’t want any maintainer to merge with the button as it just too hard to do while at the same time follow the project commit guidelines.