Multiple L2TP IPsec Users

Dear Mikrotik

Example:
I have 10 road warriors setup with L2TP IPsec VPNs using Windows 7.
I have created 10 L2TP Secrets
and 1 IPsec Peer usin 0.0.0.0/0
and a IPsec passphrase
However now comes the problem that if 1 users is fired I can change or disable their L2TP secret but now i have to change the IPSec Peer passphrase and get all 10 users resetup.

How can I setup an IPsec peer passphrase for all 10 users seeing as they are obviously on the road and their ip changes all the time?
Can you setup multiple IPsec peers using the 0.0.0.0/0 and they all have different passphrases?

How do you guys handle mutiple users and changing the pass phrase and secrets if a users leaves or is fired???

From presentatio: http://mum.mikrotik.com/presentations/HR13/kirnak.pdf

You can not have more then one 0.0.0.0/0 peer. If you configure multiple, only one will work.
–Use certificates to solve problems with one PSK for all peers.