I recently had a requirement that certain workflow transitions should fail if the assignee is one of three specific users.
Note that using this method, any number of accounts can be checked.
I am using the JMWE-provided build-your-own scripted validator, using this simple code:
let bad_users = ["abcdabcdabcdabcd", "1234567890abcdef", "0987654321abcdef"];
!!issue.assignee && !bad_users.includes(issue.assignee.accountId)
The secret that many admins are not aware of is that you can have multi-line code, using the ';' delimiter before the newline.
This works like a charm.