This is the situation:

User A is a delegate on User B's mailbox

User A has added User B's mailbox to his Outlook client (so User A logs on
to his own mailbox and has added User's B mailbox).

User A has NOT got the send-as right on User B.

Now here is the thing: User A selects the Calendar of User B from within
Outlook and sends a meeting request from there to another person.

When he does this, a report of this is dropped into the inbox of User B,
telling User B that he does not need to take any further action because he is
the meeting organizer.

This is the exact report in the mail that gets dropped into his mailbox:

"As the meeting organizer you do not need to respond to the meeting"
The From field indicates: "User A on behalf of User B".

I understand the idea behind this. User B is informed that User A has
scheduled a meeting on behalf of him. However user B does not want to receive
these messages.

How can I prevent User B from receiving these messages?

This looks like new functionality in Exchange/Outlook 2007.

I have another user who is also a delegate on the mailbox of User B and who
has (as it seems) similar rights (full mbx access, send on behalf of and no
send as). When this user does the very same thing, the message does not go to
the inbox of User B. Therefore I think this could very well be some kind of
client setting.

I know of the settings: Send meeting requests only to my delegates etc.
Which reside in Tools\Options\Delegates. But this is not it. I ticked the
checkbox "send meeting requests only to my delegates but this is not what
should happen", this would make the request go to the delegate. I do not want
the request to go the delegate. It should only go the ones who are invited,
not to the delegate, not to the user on whoms behalf is being sent. It should
just be as if User A sends somebody a meeting request which is not being send
on behalf of another user. In that case nothing goes to inbox except for the
request itself that goes to the inbox of the one that is being invited.

As I said, I have a user for who it is working. I guess it must be some kind
of client setting.

Cheers,