It's not possible for me to make any horse stand still. If they want to move, even my 430lb Shetland, they are going to move, so I never fight the movement. I have found with all the horses I've dealt with over the years, that I just let them move out around me on the lead rope but never stop spraying until they stop moving, then praise, and matter-of-factly start spraying, again. Always stay calm and "matter-of-fact," oh, and it's good to start at the front feet for the horses that are really frightened.
Yes, water in the sprayer is a lot cheaper because some horses will longe around me on the lead for several minutes, and my aim isn't always good, and it may take a few different sessions of spraying before they learn to stand, but they always figure it out. All eleven horses here, even the Boarder Horses, stand free for spraying, now, and we do it 2x a day. Several of them were scared or resistant initially, and I had to follow them around the paddock with the spray keeping them moving until they decided to stand, but now, some of them even ASK for it. The flies just make them miserable.