How to convert a multi-speed bike to single-speed with coaster brake?
The desired conversion is possible, but maybe Trek won't do it as a "default" service. You could consider going to the local bike shop and trade some of the bike parts (specially the gearing system) for a coaster brake wheel.

If your bike is like the one in the photo, you need to replace the rear hub, and get rid of the cogset, the derailer and the twist-shifter. You can do this at home if you have some mechanical skils, but replacing the hub and relacing the spokes is not very simple.

And if you're so chatty with the lovely ladies as you are when asking questions on the internet, they sure get a lot of talk! ;o)

EDIT: As Freiheit has correctly pointed, when you put a single cog and get rid of the derailer, something must be added to compensate for the almost inevitable chain slack. I suggest a tensioner like this, of DMR brand (with a bolt, to be placed in a fixed position, not a spring):



MORE EDIT!!!!
As people correctly pointed out, you CANNOT use a chain tensioner if you use a coaster brake. So the solutions are:


Use a chain tensioner but not a coaster brake;
Choose a magic combination of chainring and cog size (very difficult, might not give you a good gear ratio, cannot compensate for chain slack;
Use an adaptor (usually not the most robust solution, but ok for a cruiser if you really want the coaster brake):