Disable every possibility for a user to get a blocked file out of the Quarantine
After some fumbling around with the quarantine manager (user-portal) i figured out that there is a way to get e EXE file which is blocked (in Mailsec. - SMTP - Antivirus - Blocked file ext.) saved as an .eml-file. That's not nice because it overrides our security-policy. No user, except an administrator should be allowed to save/export/release a blocked file when it matches as an executable.