At one particular client that didn't work and he kept getting daily warnings, which in turn meant I got a daily prompt ;-) There is a global setting that affects all mailboxes fortunately and it worked at this particular client.
I tracked down an article to set the global mailbox limit higher than 2GB. Use ADSIEDIT but drill down to the attributes at this location:
CN=first administrative group
CN=First Storage GroupCN=Mailbox Store (YOUR_SERVERNAME)