Getting rid of gnats with sand

Have gnats in my veg room and plants are in red dixie cups and about 2 week in veg..I got some girls that's almost at the end of flowering now and they have sufferd these stupid gnats for awhileee and i don't want my new batch to get fucked up like them so i put about 1 inch to 1 1/2 inch of play sand in my cups...like this in the pic


Please if anyone has experience with it lemme know...I just heard that it smuthers them or something..so what the hell what i got to loseee


Thanks for any input,

I would try neem oil

Dont have any neem oil but found some on ebay will this work http://www.ebay.com/itm/12-OZ-NEEM-OIL-ORGANIC-/150404090135?pt=LH_DefaultDomain_0&hash=item2304c84917

Note: do you think i should remove the sand and maybe put potatoe slices on top soil?? or..keep sand on top?

Im thinking of getting the neem oil get dirt out and spray with the neem and maybe put sand on top? How should i do this. Thanks for any help

any got anymore input

use neem oil dude, not saying sand or potato slices wont work.. ive just never heard of it before and neem definatly helps/works