Are you using NS or DS? Are you trying to stop the Altiris Agent (NS) and\or Altiris Aclient or Dagent (DS)?
You can use a run script in DS to a "net stop - service name", you could run a software delivery job that contains essentially the same commands, you could also use the Task Server tasks that have some of these functions built in.
Really the sky is the limit, but it depends on what you have installed, and what rights you are running them under.