Yes, alpacas make good guard animals. Alpacas are substantial animals that would tower over any would-be perpetrator except for the very tallest. If they are anything like their cousin the llama, they would also be able to spit on any potential attacker and either injure them or deter them enough for the owner of the alpaca to make an escape.
Alpacas are camelids that, like llamas, hail from South America. Prized for their soft fleece, alpacas serve double duty as guard animals for smaller livestock and chickens. While they can't take on huge predators, they're quite effective at keeping foxes and smaller carnivores away from their charges. Alpacas can run quite fast, so they're able to overtake most predators invading their territory. Once they get close enough, guard alpacas kick, make loud, high-pitched noises and spit at the invader. If the fox has any sense, he gets out of Dodge before being trampled to death.
Imagine that you are attempting to rob a house in the middle of the night. You may expect that a guard dog lives at the residence, but you would never expect a guard alpaca. When a criminal attempts to burglarize, vandalize, or otherwise destroy property, the criminal is on high alert, with adrenaline pumping through the criminal's veins. If a criminal ran into an alpaca, he would be so surprised, shocked, and confused that the attempt at crime would be thwarted.
An alpacas does not make a good guard animal, because most of the time they are very tame. An alpaca mostly wants to eat and feed and sleep. They are not as loyal to an owner as a dog is, so they would not naturally protect a person. They are also difficult to keep somewhere close by.