I've got an existing Sync Rule and wish to experiment with the filters on it. I can't, the filter area is non-editable. What am I doing wrong, or is this by design?

Is it an outbound only rule? Exernal System Scoping Filters are only applicable for inbound rules. http://setspn.blogspot.com

Yes it is. Hrm, okay, thanks. I'm looking for solutions to a problem where we have some users in AD who should only exist in AD and not be propagated out to our other systems. I'll have a closer look at the MPR/Workflows and see if I can find some criteria to use to not add them to the other Sync Rules.