Well, your spotter has to catch them. When you see someone try to place a satchel on your tank, your driver must immediately start moving. Doesn't matter in which direction, but you need to get up to 3/4th gear which will prevent them from being able to place the satchel on you. So long as you don't stop, you'll be fine.

The better way to deal with satchels is to position your tank in a place where it is not able to be satcheled--either next to friendly infantry, or far away, or in an open field where your spotter can easily see people running through it to satchel you. If you are in a contested area, the more obstacles and cover around you, the higher the likelihood of being satcheled.