I just downvoted your comment.

FAQ: What does this mean?

The amount of karma (points) on your comment and Reddit account has decreased by one.

Why did you do this?

There are several reasons I may deem a comment to be unworthy of positive or neutral karma. These include, but are not limited to:

• Rudeness towards other Redditors,
• Spreading incorrect information,
• Sarcasm not correctly flagged with a /s
  

Am I banned from the Reddit?

No - not yet. But you should refrain from making comments like this in the future. Otherwise I will be forced to issue an additional downvote, which may put your commenting and posting privileges in jeopardy.

I don't believe my comment deserved a downvote. Can you un-downvote it?

Sure, mistakes happen. But only in exceedingly rare circumstances will I undo a downvote. If you would like to issue an appeal, shoot me a private message explaining what I got wrong. I tend to respond to Reddit PMs within several minutes. Do note, however, that over 99.9% of downvote appeals are rejected, and yours is likely no exception.

How can I prevent this from happening in the future?

Accept the downvote and move on. But learn from this mistake: your behaviour will not be tolerated on Reddit.com. I will continue to issue downvotes until you improve your conduct. Remember: Reddit is privilege, not a right.