Assuming you are speaking about activesync on the pc, there are steps
for shutting down activesync and rampimgr using a batch file at
http://BevHoward.com/ASync.htm#RAPI

Note that wmp will relaunch async as well as the presence of a connected
device... i.e. the ppc's usb id will trigger a program association.

Beverly Howard [MS MVP-Mobile Devices]