If program is in visible mode you can click on Email Spy Control Panel shortcut within Windows Start menu (as below):
Start -> Programs -> Email Spy -> Email Spy Control Panel
If Email Spy is in stealth mode you have to press program hotkey to run Control Panel (program default hotkey is Ctrl + Alt + Shift + E).

Click to Email Spy is active checkbox to disable or enable program work.
Then click Apply to save the change and close Email Spy Control Panel.