You can do this and make a very useful bike from it. Here's mine (which is about to be parted out and sold):
http://joshkarnesmusic.com/media/ss.jpg
I can heartily recommend 1" to 1.25" lightweight slick tires such as the Performance Forte "Slick City" like I have on my bike:
http://www.performancebike.com/shop/...tegory_ID=5430
I pump them to 90psi (5psi over the recommended limit) and I have 1500 miles or so on them with only one flat (glass) and no other issues, probably have another 1500 miles left in them.
For my bike, I also took the fork apart and took the spring out, replacing it with a length of aluminum tubing so it becomes mostly rigid. My fork was an elastomer top out bushing on top of a coil spring so I just replaced the spring with the tubing and then the elastomer gives it about a half inch of real stiff travel. It works great going off curbs etc. Perhaps you can come up with something similarly creative for that fork. Maybe you can download an RST service manual for the fork telling you how to change the spring for a different rate of spring, and follow those instructions and replace the spring with something solid like a piece of PVC pipe.
The alternatives are either to put up with the fork compressing on you all the time (kind of annoying) or replace it with a rigid MTB fork. If you can find a rigid MTB fork, it may be expensive, heavy, or both. And it may also be considerably shorter in crown to axle length vs. the suspension fork at the recommended sag. This will affect the geometry of the bike, most likely in a positive way for riding on the road, but mostly it will make the head and seat angles at least a degree more steep if you get one of the common cheap steel forks, maybe more. The bike probably has 71/73 angles and could stand to be 72/74 or even 73/75 if you are only riding on the paved roads.
With 44/16 gearing I can spin at about 25mph on this bike. As you can see I have the seat post past the max extensioin, seat all the way back on the rails and a super long stem on the bike. If this frame were a 20" instead of an 18" I would probably keep this bike to ride on rainy days. I like riding it a lot. The bike is about 21 lb or so.