Rulings

An effect may redirect Excruciator’s damage.
Excruciator can deal damage to creatures with protection from red. (It can’t block creatures with protection from red, though.)
Damage prevention shields that would prevent this damage aren’t used up and they stick around for the next time something would deal damage.
A creature dealt lethal damage by Excruciator can be regenerated.
Replacement effects that don’t use the word “prevent” can replace Excruciator’s damage with something else. See Phytohydra and Szadek, Lord of Secrets, for example.