"it annoys me when people go out of their way to be unhelpful"

[mwtzzz]

Some help translating this: "it annoys me when people go out of their way to be unhelpful"

"go out of their way" means "deliberately". I'm not sure how to express this, other than to use the phrase "a propósito":

"Me molesta cuando la gente, a propósito, no es servicial."

[AngelicaDeAlquezar]

Your sentence is fine and would be understood.

As a side note: some people, tend to confuse "servicial" with "servil", which has a negative meaning, as that of someone who has little or no self-worth, and that is why they're ready to make themselves useful for others, even when they're not needed. They're not a majority, but I tend to avoid the word to avoid funny looks.
An alternative is to change it for the verb "...no ayuda (a otros)"

My proposal:
- Me disgusta la gente que se toma la molestia de no ayudar / ...de estorbar (they don't offer help, and they become actually an obstacle).
-> It's an unusual formulation and the sarcastic charge is almost aggressive, but I think this is pretty similar to the idea in English. (?)

[mwtzzz]

Seems like a pretty good translation that gets the spirit of the English phrase across.
"Go out of their way" means someone is "taking pains" (an equivalent expression) to do something:

"He is taking pains to make sure she doens't get what she wants."

[AngelicaDeAlquezar]

Yes, that is why we don't use it normally in first person, to avoid sounding conceited:

- Me tomé la molestia de saludarte. -> I'm so important, that I could have just ignored you, but I took pains to say hello to you.