How to block a plugin from running in one theme only

I’d love to know how to block one plugin from loading/running/etc using a theme’s functions.php file. The reason for this is that I have a custom setup that detects browser user-agent and dynamically loads a theme based on that UA. Well, one of the themes, I need to block the original plugin from loading/functioning, then I want to include the plugin’s code in my functions.php file so I may code the plugin to work differently for just tht theme. Any ideas?