Steve#1, welcome to the forum! A quick way to stop it is to not run the pump . However, that's a bad choice, as you want the pool water to circulate when, as long as the pool isn't closed for the winter.
As a 'stop gap measure', you can either put a valve on it, as AL suggested, or plug it when you're not backwashing or wasting (there are a couple of ways to do this, depending on how the backwash is plumbed, though the simplest is to unscrew the backwash adaptor and put in a SP 1022-C 1 1/2" threaded plug w/ O-ring). There is something wrong on the inside of the valve - it could be a torn or 'buckled' 'spider gasket' (aka wagon wheel) or a broken 'channel' that seats the gasket. The thread Al alluded to is http://www.poolforum.com/pf2/showthread.php?t=1331. Give it a read and see if the advice there fits your situation.
When you're ready to take a look inside the multi-port, use a #3 Phillips-head screwdriver to remove the 8 screws (keep your hand under the locknuts, so you don't loose them). The top has a notched edge on one side, so as long as you match that up, you'll be able to reassemble it in the correct position, if you need a new gasket the part # is SP 710- D - if the channels in the bottom part of the MP are cracked or a section is missing, you need to get a SP 710 [u]R[/] body for it - the "r" designates that it's for a DE filter.
I hope that Ive properly explained how to see if the valve itself is broken, (while I was looking up the link to what Al said, I came across a thread which tells you how to proporly backwash your filter, http://www.poolforum.com/pf2/showthread.php?t=3613 - post #6.
I welcome you again to the forum, should you ever need any help with your pool, please ask us!, and we'll help you as much as we can (and that covers just about everything )