I've had a similar problem where users who used to be in a specific Entourage-users's delegates list 6 years ago were still getting forwarded meeting requests, even though they'd been removed years ago. These particular users lived with this annoyance for quite some time before reporting the problem, and it was only after we moved to Exchange 2007 and the Entourage user had some other problems that they mentioned it to me.
During the troubleshooting process I learned some things. First of all, if you have any Entourage in your environment it is a very good idea to download MFC MAPI from Microsoft. This Windows utility gives you a pretty raw view of a user's Mailbox, and is the only way to fix some problems.
When you set a delegate, it creates a hidden rule to handle the forwarding of the meeting requests. What I didn't know is that earlier versions of Exchange put that in a different spot than Exchange 2007 does, but Exchange still runs the old rules if they are still present. In my case, there was a delegate rule still present in the old location that was still being run.
In MFC Mapi you get a top level view of a user's mailbox. One of the items in there is called "Schedule". If you open "Top of information store", the rules are kept on "Inbox". In each folder there is a Rule property that contains the rules for that user. The one at the top of the mailbox is the old one, the Inbox in the "Information Store" is the new one. In my case, the rule was hiding in the old location. One that Even Outlook 2007 couldn't find.
In this screen shot you can see a rule called "PR RULES DATA". This is the actual rules data. To decode it, select the 'Schedule' item and click "Actions" on the MFC MAPI menu. There is an item there called "Display Rules Table". If your problem is like mine was, there should be a rule in that top-level Schedules directory. Hidden rules start with an underbar. Right-click on the offending rule, and click "Delete'.
Like Regedit, you can do serious damage in MFC MAPI if you're not careful.
A second more obscure possibility for your problem is that User B has User A's X500 address associated with it. I'd be surprised if this would be the case, but it could happen. Entourage is more sensitive to that sort of thing than Outlook is.