The most impact is the rider. ;) Most riders become stiff because they 'lock' their joints in order to keep the horse from getting faster. If a rider becomes stiff somewhere in the body, all the joints have more tension and as a result the rider cannot move in rhythm with their horse anymore.
So stay 'elastic' in your body. If you want to slow down the horse you can influence his pace by moving a little bit behind the movement to slow him down. In order to achieve that, you must stay flexible yourself.
How to stay flexible and relaxed?
A good first step is to stay conscious of your own breathing. We, people in general, are tented to hold our breath if we do something excited or make an effort. Holding your breathe makes you strained, so keep breathing. Be aware of the flexibility of your joints (neck, shoulders, elbows, wrists, hips, knees, ankles etc)
Then follow your horses movement FIRST, before you can slow them down with your seat.
Horses do listen well to the riders seat. If you exhale and relax, most horses will respond by slowing down because the rider relaxes. Make halfhalts with your body.
If you use the rein (a bit), make only a half halt and don't "pull" (I don't know an more subtle word in English at the moment, sorry for that) until the horse has slowed down. The half halt is a signal to communicate to the horse, the reins are no brakes. So harder pulling does not mean hitting the brake harder.
Frisians have a tendency to go against pressure. So pulling the rein will not stop or slow down the horse, it will make them go against your pressure. Use halfs halts instead and reward for the slightest try!
So, if your horse is responding by slowing down just a fraction, reward! Then ask again untill you can let him walk. You can reward the horse by relaxing your body and move in the same rhythm. Then ask the question ("slow down a bit please") again.
I notice that is it quite a challenge to put this in writing, in English. I just immigrated to Canada and my Dutch is still a little better than my English. ;)
Besides that, it is also hard to give advise through the internet if you have not seen a rider. But the above is the most common solution to (start to) resolve the problem. Other reasons for this behaviour could be: saddle doesn't fit, teeth are hurting (most Frisians have not so good teeth because of the small gene pool), bit is hurting, too much energy due to diet/lack of exercise and more.