Couple ways to achieve you're desired results, here's 3 ways to do it:
Free way: Remove front shifter, front shifter housing, remove big and small chainrings, shorten chain, adjust lower limit on the front derailleur so it's aligned with the middle ring. Might not work great, but it should work.
Cheap: Remove front shifter, front shifter housing, remove all chain rings and add single speed front chain ring of your choice in the middle ring spot, shorten chain, adjust lower limit on the front derailleur so it's aligned with the middle ring.
Recommend: Remove front shifter, front shifter housing, remove front derailleur, remove all chain rings and add single speed front chain ring of your choice(I like the e13 guiderings, they offer odd sizes not just even if you know exactly what you want) in the middle ring spot, shorten chain, get a 1 speed front "xc" chain guide like the MRP 1.x, e13 XCX, I think paul makes one also. e13 XCX gets my vote but it's also the most expensive depending on which configuration you go with.
With all these setups you don't have any sort of bashguard or extra protection, I dunno if you smash your big ring a lot (keep in mind you will have a lot more clearance if you run a 32T front chainring as well, so if you don't smash your big ring alot I wouldn't worry about it). I run a full chainguide with a bashguard on my stumpy evo, but I've put some nice sized dents/scrapes in the bashguard that could have taken out my chain/chain ring..