If Too many s sensors nearby is displayed at the start of your exercise, your Polar RS800 training computer is within the transmission range of two or more Polar s3 stride sensor W.I.N.D. / Polar s3+ stride sensor / Polar G3 GPS sensor which are saved in the training computer's memory and both/all of them are moving.
If you have more than one stride sensor / G3 GPS sensor in the training computer's memory (i.e. you have 'taught' it more than one sensor), make sure that only the one stride sensor / G3 GPS sensor you wish to use is in motion when starting the exercise. Your RS800 should be locked to your stride sensor / G3 GPS sensor before you can move any other taught sensor. This way your training computer will only acknowledge the signal from the stride sensor / GPS sensor you want.
If you wish to have only one sensor in the training computer's memory, go to Settings → Features → S sensor and select Remove all. Now teach your stride sensor / G3 GPS sensor again.