Why would you want to prevent them from actioning their tasks during deployment?

The tasks that are already created will continue to run on the previous version of the process and you publishing a new version shouldn't affect the users existing tasks.

Isn't there a publish mode where it brings all the existing in-flight processes to use the latest version of workflow definition?

Would that mode cause any corruption to the data if the user are working with instances of request while a new process is being deployed?

I guess there is no facility to bring a specific process "off-line".

I'm actually planning on moving the inflights to the current version once its deployed so thats why I dont want them changed. I also want to limit server activity and general variability while I'm deploying the processes. Otherwise your point is an excellent one.

Okay so I'm assuming you're going to use the Live Instance Management APIs to move the running instances to the new process version?

As to limiting server activity I was wondering if before deploying the new version if you, first remove all start permissions so new instances can't be started and  "Stop" all the running instances.  This will cause all the worklist items to disappear from users worklists and they cannot action them.  After deployment and using LIM you could then reset the start permissions and "Start" the process instances so that the worklist items will become available.  Note that I haven't tried the stopping and starting of instances with LIM so please insure you do some testing in your development environment.

I hope this helps.

Tim