I don't think there is any way to do this only when installing ActiveX, atleast not without have the actual ActiveX installation to disable UAC, which i doubt is possible.
It is not any good idea to disable UAC in general, as it is their to protect you from running unwanted executables that may harm your computer.
Despite that, then you can find the settings for UAC in the registry:
Go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System
Find the DWORD ConsentPromptBehaviorAdmin
Change the Valuedata
to 0
This way you can disable UAC without rebooting.