Ultimatums really don't work. You can't force someone into changing in ways you want to by applying pressure. In some cases, they may see what they risk losing if they do not give up a certain destructive behavior. But usually it is too late. Besides, do you want someone who merely modifies their external behavior because they are afraid of what will happen if they don't, or do you want someone who has a true change of heart, without any external pressure from you or anyone?
Change is hard to come by. If someone who has been a habitual offender suddenly promises that everything is going to be all better and that they going to seek help, be very slow to believe them. Many times, people appear to want to change when really they are just panicking because the status quo is changing, they are losing their security blanket, or they want company in their misery. Most people don't change that much -their basic strategies for coping and relating, the preoccupations and allegiances of their heart, etc. God can change anyone, and he does, but as a general rule people don't really change all that much.
People won't change their lifestyle until it is their aim to do so. You cannot hash out a "plan" for someone to fix their life and expect them to follow it unless it is their plan or unless they asked you to help develop a plan for themselves. If they aren't taking real initiative to follow through, they won't. And if you keep trying to do this for them, you are just enabling them and allowing them to bring their crazy roller-coaster of drama into your life... and for what?
Boundaries are an unfortunate necessity in life. People don't always manage things well, and they have their own baggage that they carry around everywhere they go. If you don't keep watch over your domain and the boundaries of your domain, if you keep trying to just be nice and accommodating, you are going to end up allowing those people to share their bad with you, and it will indeed infect you life and everyone and everything in it. They may want to get close to you to hurt you, or they may just expect that everyone else should join them on their drama-filled roller-coaster of misery. Either way, learning to say "No, and you need to stay out" doesn't feel good, perhaps, but it is necessary for the sanity of your own life and those in your life that you need to love and care for.