When I look at my antivirus software I see the scanner runs as a windows service under the system account, it also has the CanStop set to false. The UI is a regular windows app, and it doesn't matter if this is closed/reopened because it is just a user front-end to the underlying security services. Whether it has additional locks I don't know, but you could try and see how that works.
If your AV by any chance is a windows form then one option is to just hide you app from the applications listing in task manager by setting the forms ShowInTaskbar property to False.