It’s in the voice allocation mode (the mouth button).
Actually this doesn’t seem to stop the arpeggiator; when I was remaking the Covenant track last night, I noticed that if trigs are long enough to overlap the next one, the notes from both trigs are added to the arp until the shortest trig ends (at least this is how it sounds to me). Could be a feature, but it’s tricky to prevent—all note trigs must have their lengths set precisely, otherwise overlaps will occur. This makes things a bit more difficult/frustrating when it comes to using trig conditions on a poly/arp track. Perhaps there’s scope for a “cut previous note” option for trigs in poly/arp mode?