Bicycle Tutor

How to convert a multi speed [hand braking] bike to a primitive pedal brake bike.
It's just unconvertible to ride the bike this way after many years of pedal braking bike.

You'll have to replace the whole drivetrain: replace hub (or entire rear wheel) and chain, remove the front and rear derailleur, remove the cables, buy a new shifter, route new cables on the left side (without cable stops -> you'll have to improvise). I'd probably also get another crank set, since the chain rings are designed for a narrow chain (3/32 inch) and internal gear hubs (as far a I remember) usually use 1/8 inch. Note: With a coaster brake, you _cannot_ use a rear derailleur, it will get bent the first time you brake.
Other than that: get another bike (might be less expensive) or get used to it, yes, this will take some time. Sorry and good luck.

Joe_W is accurate about doing a thorough conversion to a multi-speed bike with an internally geared hub with coaster brake. However, I suspect you might be looking to just swap the bike over to a simple single speed coaster bike. I've actually seen this done a lot. (see: http://www.atomiccycles.com/coaster.html)
Hardest part is getting the coaster wheel. If you have 26" wheels, there are a lot out there available. Other sized wheels are harder to find and more expensive.
Coaster hubs are usually narrower than geared hubs. But there's normally enough axle that you can add some spacers or flex the frame to fit (only if it's steel)
Many do use 1/8" chain. But you can probably use a 1/8" chain on your existing crank without any major issue. (possible issues there depending on your crank though)
Lastly, might be hard to do if you don't have horizontal dropouts so you can adjust chain tension. There are "tensioners" you can get as a work around, but they're not ideal when you're using the chain as part of your breaking mechanism.
I say go for it if your bike fits the criteria. Or, find a cheap used cruiser, put some faster tires on it, and call it a day. (see: craigslist)