First, clamscan will stop spawning when you stop postfix. But it may take some time, mostly about 5 - 10 minutes.
The emails to be scanned are most likely in the mail queue. You can manage the mailqueue with the postqueue and postsuper commands.
You should try this to get down the load from clamav:
clamd is much faster then the current clamscan solution. Sinvce the clamav developers introduced clamav 0.90.x, the antivirus scanning is producing a much higher load. the problem seems to be known according to the clamav mailing lists and the clamav developers recommend to use clamd. There is also some patch which I hope will be integrated in some of the next clamav releases.