Finally thought of a good use for spring-loaded mouse traps... set them and put them in your pockets to catch a thief. Just don't forget you put them there.
In all honesty, addicts are a lose-lose situation unless they are ready to or have already gotten help for their addiction and even then, they live from hour to hour battling temptation. That is why so often they look to family to help them out because of the whole enabling/unconditional love aspect in most cases. She needs help beyond what you and your other roommates are qualified to give her and it sounds like she has learned and worked her method of "dealing" with confrontation so take the "nothing personal, it's business" route and just let her know you think it is a poor fit, that she needs to start looking for another place to live and that you guys will start interviewing new potential roommates.