Is there a way to disallow Stash merges
for failed pull request builds with
Stash pullrequest builder
For answering your question, yes (but not automatically or programatically)
As you know that when a developer creates a pull request he mention some users as Reviewer. You can show the reviewer that the build is failed or not as added in comment. Then its reviewer's work to reject the merge. you automatically not able to reject the build.
Also if your are using SonarQube then you can also show the sonarqube result also in pull request