I am frustrated and don't know what to do

So what am I supposed to do when a person I am helping both doesn't completely understand me and is very stubborn in wanting what they want even though he/she doesn't know what they want. Yeah, if that is confusing to you just think that it is even more confusing to me. It's even more frusterating when you try to communicate and the other individual just says no to EVERYTHING you say even if you say "lets get ice cream"  I understand he/she is bored and did a great job at being patient and has a disability but how am I supposed to reward him/her when he/she has sudden bad behavior right before we were supposed to do something fun? I can't so now the individual is upset bored and I don't know what to do. God show me the answer please.