If you want to try single speeding a MTN bike, particularly one with vertical dropouts, find a gear that works for everything and don't leave it. Once you know this is the one and are commited, use the range set screws to fix your front and rear derailleurs in place. Then go ahead and remove cables, shifters, housing, etc. You can leave the front derailleur on as an anti-jump device and you have just turned your rear derailleur into a singleator. Anything else you do from now on is for weight savings or aesthetics. GO HAVE FUN.