The short answer is yes... but will they?
Do they spit at people? 99% of the time the answer to this one is no.
Spitting over food
The most common time you see llamas (or alpacas) spitting is when they are competing over food. For people who are lucky enough to spend a lot of time with llamas this is the most common time they have accidentally been spit on. The llama is telling its herd-mate to back off and the person happened to be in the wrong place at the wrong time. With a bit of awareness and some herd management this is avoidable.