In google searches I find the following 2 parameters for Stash, but are they valid in Bitbucket Server and where would you put them?
You can disable forking altogether by adding 'feature.forks=false' into your stash-config.properties file.
The ability to restrict creation of personal repositories shipped in Stash. To configure it, set feature.personal.repos=false in stash-config.properties
Thanks,
Mike