The question says it all. Is there a way to perform an action before a merge? I'm guessing there's a way to make use of a pre-commit hook, but I'm not quite sure.
You can try using the prepare-commit-msg hook. The second argument will be merge "if the commit is a merge or a .git/MERGE_MSG file exists". A non-zero exit status will abort the commit.
I don't think this will work with a fast-forward merge, since there won't be a commit message.
More info on hooks: https://www.kernel.org/pub/software/scm/git/docs/githooks.html#_prepare_commit_msg