To map controls on the Kontrol 49 to Ableton Live while Reason is running, go to Reason Preferences > Control Surfaces and Keyboards. In the list of controllers, find Kontrol 49, and uncheck the "use with Reason" checkbox. This will disable the Kontrol 49 in Reason, but will free it up for use with Ableton Live. 

Before you leave the studio, please return to the Reason preferences and recheck this box, so the connection to Reason will work for the next user.