I believe that there is no option at present to have the AV system power down the machine on completing the scan,
However you could consider setting up a power management function to shut down the machine at a set time, this would then be independent of the AV scan, thus preventing a user launched scan from shutting down the machine if that was done (depending on your settings etc)
Therefore you could approach this from this angle. As an example,
1. AV scan starts on the machines, and takes about an hour to run, set to do this at 8pm
2. @ 9pm the power management module policy kicks in and turns off the machine
You would then also have a second policy for this that would be one that does not turn the machine off for those users that access their devices after this time, or have actions on their machines, that means they need to stay on.
Or you can apply this so this happens on a Friday evening thus turning the machines off for the weekend and saving on power costs that way.
I know that this is not a direct answer to your query, but hopefully it will allow you to consider a company policy that you can adjust to meet your needs to scan and turn off machines.
This is a great suggestion.
In LDMS 9 there are process sensitive triggers for Power Management that may make this even more feasible.