Obviously, the matched bets cannot be cancelled but those yet to be matched can be. However, as the whole purpose of the trigger is to match all bets it's usually better to let the race run its course. I have, occasionally, placed additional bets to reduce my losses when it is clear that one of the unmatched runners seems to have a good chance of winning. So, I use the dutch chaser trigger as an automated trigger but monitor it "manually". I hope this makes sense.