How to prevent merge request from a webhook

I feel I might be missing something obvious but I can’t see any direct way to use a webhook to vet a merge request and refuse to merge it if it doesn’t meet with our requirements. I have configured a githook to do this but the webhooks seem easier to use and I am using web hooks for all of the other integrations that we need. I sort of expected that returning an error status to the merge request merge webhook might do the trick but it is explicitly stated that the response status is always ignored.

Thanks
Andy