How do I remove my bicycle handlebars that are connected by a single rusted bolt? I'm trying to remove the HANDLEBARS from the stem of my bike and the handlebars are put into the stem and secured with a bolt. However, the bolt is rusted and will not budge no matter how hard I go at it. Thanks!!
Sounds like you have a Quill Stem, which has a single large long bolt going down into the steerer tube, which is part of the fork.

Here's a cutaway showing what's inside.


Generally speaking they're fairly corroded over time.  The thing labelled Clamp in the diagram is more normally known as a wedge or expander.

So you need to get some penetrating oil into the threads in the wedge.  This is not possible from above.


Remove your bike's front wheel and put it aside.
Flip your bike over, and identify the dark hole in between the fork legs.   You may need to remove any mudguard or fender too.
Squirt some penetrating oil down this hole and let it sit overnight.  Probably a good idea to put some rags, paper or basin down to catch any oil that comes through.  Some will sneak around the sides.
After a day of sitting, try and undo the bolt.  


If the bolt moves, undo it about 2 full turns and then give a gentle tap with a hammer downward.  This will knock the wedge loose.  If you remove the whole long bolt, the wedge will be left behind.



The reason heat didn't help is because the head of the bolt is a long way from the threads, and the heat won't carry that far.  Applying heat to the outside of the bike won't really help either.