I'm fairly sure that Debian uses the PoPToP pptpd, which in turn uses pppd. The pptpd process forks for each new user connection so you should just be able to kill the 'pppd' process related to the user you want to terminate.
The process list doesn't show the username associated with the pppd process for a given connection, so you'll probably have to use the last command to figure out what IP address user the user logged-on from and then kill the appropriate pppd process. pptpd, by default, updates the wtmp file with logons, so last should show you from what ip address a user logged-on from. Then it's a matter of grepping the process list for that pppd instance.