Exchange 5.5 Send on Behalf

In Exchange Admin go to the user(s) she is sending on behalf of and within the mailbox properties choose the Delivery Options tab and check she is not listed in there.
Also check under her mailbox on the permissions tab and compare her rights with a normal users to make sure she hasn't been granted anymore.

Mozter
On the Delivery options she has no-one listed, and the other users also does not list anyone, so No one has permission to send on behalf of others.
You stated check under her mailbox on the permissions tab and compare her rights? Where would I find this. If you mean Administrative Users, she just has Domain user just like everyone else?

Have the users she is able to send on behalf of given her permission to in outlook. If so, they just need to revoke this permission.

Do you mean "Send on Behalf" or "Send as"? "Send on behalf" permissions means the recipient will see something like "from Jane Doe on behalf of the Executive Vice President". "Send as" means that Jane Doe can send a message that looks like it came directly from the EVP. The "Send as" permissions are located under the Permissions tab of the recipients mailbox (This tab is hidden, unless you expose it in Exchange Admin through Tools -> Options -> Permissions -> Display permissions for all objects).

Under her email tab, Permission, the send as box is not checked,

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