There is one way to protect against this - spam click the spell button until the attack lands. That way the defender won't have time to cast another spell than purification that you'll already have cast another protective spell.
Issue is that people can only keep clicking on 1 attack. So on conquest attempts, clearing attacks can be reduced to nothing, while the attacker is clicking on CS to keep that one safe. Plus on most conquest attacks, there will be breaks in clicking where the defender can get some spells in, as the attacker can't fire off attacks/support at the exact time as continuously clicking on the spell button.
Also players have been banned (attackable) for doing that also not really great out come, plus the clicking thing is dull. Not too bad on worlds with fast unit speed but personally I wouldn't want to sit and click for a few hours on a speed 1 world.
However it is good advice as it is the only option to keep units safe.
I don't think a limit is needed, this is a strategy game and the number of spells that can be cast on an attack should not be limited, as this is part of the defender's strategy.
I agree to no limit, but I miss the days when spells could protect attacks.
I detest purification, when combined with favour farming and tokens, it becomes way too overpowered imo.
Personally I'd love to see purification be scrapped but if it has to stay, I think it would be better to make it cost more.
Also suggest if a new god is ever added, make 1 spell protection from purification or add a hero that has a ??% chance of deflecting spells from the defender.