Here are a few commands useful for pptpd server admin:

Find out who is Online:

last | grep ppp | grep still

Get a list of user’s connection time:

ac -d -p
If you don’t have ac on your server, run:
apt-get install acct

Disconnect a User

1. find out user’s ip (we know it from “Find out who is Online” part)
2. use command below to find out process id
   ps -ef | grep | grep pppd
3. end user’s process
   kill <process id>

Further Reading:

pptpd server – poptop document