Excluding groups from creating an issue

[Az Henderson]

I'm having difficulty in finding how to exclude a group (or groups) from creating issues in a Jira project.  As background, I'd desperately prefer not to create an inclusive group of users and give that the "Create Issue" permission, as the administrative overhead of maintaining this additional group would be excessive.  I've exhaustively searched the forums & interwebs for a solution, without success.

The use case I'm looking to implement is:

"Everyone can create issues, so long as you're not in Group A, Group B, or Group C."

Logically, I'd like to put a Validator on the Create transition to perform this check, but I cannot find the appropriate syntax (if it even exists) for where to insert a "not" (or "!", as the case may be).

e.g. - I know that something like this will work for an inclusive check & return a Boolean:

issue.get("creator")isInGroup("Group A")

but I need something more like this (yes, I know both of these code blocks contain errors):

issue.get("creator")isNotInGroup("Group A")

or

!(issue.get("creator")isInGroup("Group A"))

Alternatively, if someone is able to help out with how to invert the Boolean value received from the inclusive check, that would work just as well.

We currently have Scriptrunner and JMWE installed.