Mac OSX VPN Server has reached capacity...

I have a Mac OS X Snow Leopard server running a VPN service to support Mac OS (L2TP) and Windows (PPTP) remote clients. The VPN service is configured to allocate about 20 IP addresses to PPTP clients with the usual concurrent number of users around 5-10. Everything operates as expected, however about once a month to 6 weeks, users start getting the following error message when attempting to connect from Windows (PPTP) machines:

Connecting to xxxxxxx.xxxxxxx.com using "WAN Miniport (PPT)'...

Error 807: The network connection between your computer and the VPN server was interrupted. This can be caused by a problem in the VPN transmission and is commonly the result of internet latency or simply that your VPN server has reached capacity. Please try to reconnect to the VPN server. If this problem persists... blah, blah, blah.

On previous occassions I've taken a look at the OS X server (locally) and determined that the PPTP connections that have been established aren't being released. There appears to be no function that allows me to manually/ forceably release a connection without restarting the service thus disconnecting existing users. It would seem appropriate that when a user disconnects their VPN session, that that session will be released automatically.

Am I missing a setting or process to enable that action?

Thanks in advance.

Tony