Sorry to say but i think every one is right and there is not much u can do about it. Look at it this way your dog must really enjoy doing agility or it would not get so excited.

My collies bark i have some on a black dog haltie's and some in harnesses all this does is give me more control.

I use all sorts of things with mine one is a stupid merle so just a quick check and a ah! stop's him for 3 secs, one loves her toy and can squeek it to get her attention then play tuggie, my pup is being corrected and made to sit and that shuts him up but i have friends that leave them with some else, in cages and vans, and use a high pitch spray can called correct or something like it.

Don't give up or worry we all have the same problems, keep going once you feel more confident you'll love it asmuch as your dog, and if it makes you feel any better my merle use to bite me!! luckly he has grown out of that now he understands this agility game