There's no way to "prevent" policies from running, but there are a few things you can try.
1) remove the policy files before deploying the image. While the system is shut down for capture (or deploy) remove the policy files from the system. That way, nothing "saved" can fire while the deployment script finishes. New files can come down granted, but nothing saved will fire first.
2) prevent communication for a time. You can actually delay communication with the NS for a time, but unfortunately the delay is random. <sigh> It's configured in the targeted agent settings. The delay though might buy enough time for completion.
3) call support. We may have a file for you that can be used to further delay things. I'll explain.
We've discovered that we forcibly start the agent and get things going as SOON as the system reboots. However, we've also discovered that during MiniSetup, the services are not supposed to be running, and ours is. What this creates though is a situation where tasks (and policies) are firing during MiniSetup, and messing things up. For instance, let's just say MiniSetup is joining a workgroup, and at the very same time we're running a config task to join a domain. We finish, MiniSetup freaks-out because the system it thought it was renaming no longer exists, and dies.
The fix was to not start the agent until MiniSetup does. We have a file we may be able to give you to help you avoid this problem.
That said, it will keep the agent stopped, so no tasks will fire, OR policies. But if MiniSetup is kicking off the task, it will buy you time there.
Best we can do for now.
I repeat, there is currently NO WAY to prevent a policy from firing at the same time a task is firing. There is talk about doing something like this, but it hasn't been done and I don't know when/how it will.