Q:

How to prevent wasting time with some users

How can we avoid wasting time with users that don't deserve our attention, specially new users? Do I need to always open the user profile to check if he is a bad/good user?
Example:
Question:
AdMob Ads on Android Canvas GamePanel
User:
https://stackoverflow.com/users/2399229/user2399229
I spent some time answering that question, but then the user insulted me (comment already removed). After that I opened the user profile and saw all those negative questions on his profile... =(

EDIT:
After all these negative points from my question, now I'm becoming a bad meta user, lol!
But I really liked the two answers so far! What really upset me was the insult itself...

A:

New users, even bad ones, deserve our attention. 

If they haven't found the FAQ, we can point them to it.
If they ask bad questions, we can help them understand how to ask better ones.
If they post offensive things, we can flag and/or edit the problem.

Everyone was a newbie once, and mistakes are to be expected. Part of our job is to help those bad users become good users.
If the user does not learn or is being intentionally disruptive, an automatic ban will eventually be enforced.

A:

George's answer is an excellent point and a good answer. I'm going to approach this from a different angle, as well.
You should generally judge whether to help with a question or not based on the question, not the user. If it's a good question, give it a good answer. If not, don't. Try to help if you like, or just leave it alone if not. Don't be afraid to vote (down, close) or flag (if appropriate), if you wish.
Note that sometimes, 'bad' users will spontaneously become 'good' ones. People are capable of learning - sometimes at an alarming rate. If someone's post is a good one, answer it; even if their previous contributions have been 'bad', you'll be reinforcing their new, good behavior.

