Choose all pads that shall choke each other by ctrl- (win) or cmd-clicking (mac).
Click on the edit page. In the edit-page, choose “main”. There you see the maximum number
of voices and the exclusive-group on top of the waveform-window.
Set the exclusive group by hovering your cursor over it and scrolling.
Or double-click in it and type a group number.
If you want that the sample chokes itself too, set the maximum number of voices to one.
Cheers.