Are you saying that it regains focus (i.e. reopens)? I'm not sure how to interrupt "takes over priority".
Stealth mode should only regain focus if a team message or email is received. If it is doing this for a call it's a bug. From the CAD user guide:
The window appears as an icon in the system tray (lower
right-hand corner of your desktop, next to the system
clock) and doesn’t appear unless double-clicked or if you
receive a team message or an e-mail.
Speaking of bugs, I strongly recommend that you place a PUT order for 8.5 and apply 8.5(1)SU1. The 8.0(x) release is a train wreck and has caused multiple CAP cases.