So you notice the enemy is attempting to spike to one of your group members. You have 5 to 10 seconds to react before the enemy kills your teammates. Here's how to counter a spike attempt.
1. If you can detaunt your attacker, do so. You might be knocked down or silenced and unable to do so. Call it out in vent ASAP if you get hard CC'd. Do not attack until you can get a Guard. If you are Guarded, detaunt is not needed.
2. If you are a tank, get a Guard on the spiked target if you can get in range, even if that's another tank.
3. If you see backloaded DoTs like Burn Away Lies or Word of Pain, cleanse them if possible. Cleansing Wind is big help, especially if you have it maxed.
4. If you are a healer, get heals on the spiked target. Absorb -> HoTs -> Hard healing.
5. If you have a melee train on you, either an AoE stagger or knockback can handle the train.
6. Use your potions and morales if needed. You can fire absorb + crafted heal + RvR heal potions all at once.
7. Kite if you notice before it starts. Try to kite towards your DPS if you are not already with them.
This game is about spike and counter spike. Master managing incoming and outgoing spike damage and you will be nearly unstoppable.