When you have finished with the PPP link, use the standard ppp-off command to shut it down (remember - you need to be root or a member of the PPP group!).
In your system log you will see something like:-
Oct 21 16:10:45 hwin pppd[19873]: Interrupt received: terminating link Oct 21 16:10:45 hwin pppd[19873]: ipcp: down Oct 21 16:10:45 hwin pppd[19873]: default route ioctl(SIOCDELRT): Bad address Oct 21 16:10:45 hwin pppd[19873]: fsm_sdata(LCP): Sent code 5, id 2. Oct 21 16:10:46 hwin pppd[19873]: fsm_rtermack(LCP). Oct 21 16:10:46 hwin pppd[19873]: Connection terminated. Oct 21 16:10:46 hwin pppd[19873]: Exit.
Don't worry about the SIOCDELRT - this is just pppd noting that it
is terminating and is nothing to worry about.