How to DECREASE the WiFi rang of my Router (att uverse motorola router)?

ii do use the wifi on my iPod touch & laptop so i dont turn it off. router in my rom and its annoying to have my sister and her friends to use it because they freaking leach off of it like no tomorrow so i lag a lot online when playing online games on console.YES i have a password on but i need to give them it cuz my lil sis cry to mommy that i dont want to...so i need to decrease rang by a lot so their wifi signal sucks. i have a att uverse motorola router
ps sorry for typos i was typing fast

this Dawn from WasteLAN in Tampa?

it's Jay