We don't like to merge branches into master without reviewing the merged output before the merge.
So our flow for merging branches is as follows:

1. A feature branch is approved (pull request needs to be merged)
2. git merge --ff-only
3. If merge failed, switch to the feature branch and merge from master, wait for approval of pull request with the additional merge commit, start over

But it seems that there's no way to block a merge of pull requests in BitBucket, only if there's a merge conflict.
Personally I use BitBucket and is happy with it, but unfortunately this collaboration feature is a showstopper for us in using BitBucket in our company.