I can see a little icon () like this next to your name. Clicking on this gives me the option to PM you. That option seems independent of whether or not the user actually has enabled the PM system for their account. I dunno if that's a bug in the forum software or simply a problem with how we've got it set up - you'd think the two would correlate. Until that changes (if it's even possible) AFIACT the only reliable place to find out if the user has PM's enabled is on their profile page.
If don't think it's a problem with a user blocking PMs for others simply because there see to many users I see this problem with and I can't have upset them all (can I ?) .