This is one where I've been around the horn. The original LLC partners can get a Temporary Restraining Order to prevent him from opening his doors or doing business for ten days or until some short period of time has passed so that they can go through a Preliminary Injunction Hearing. The kinds of things they can bring to this evidentiary hearing are ANY conversations with clients of the LLC where he mentioned the new company because those conversations can be held as opening a sales pitch to steal the clients of the original LLC. There can be NO retention of anything from the original company... no secrets, no manuals, no how-to notes, no cell phone, phone book, no address list, no client list, because these are perfect proof of the dissociating partner attempting to use company information against it... and even if there is no agreement, the company has a right to keep its client lists protected by fiduciary duty that partner owes his other partners. Also, if he is going to make a move and sign contracts or do anything REAL toward creating the new company, he should TELL his partners... because he still has that fiduciary duty/duty of loyalty to them until he is totally dissociated. The hard fact here is that the original LLC only has to show that they stand to lose business to this person... and while his rights to do business are inviolable in some states, his rights to compete with his former partners can be limited by a Preliminary Injunction just because he didn't go beyond following the rules.

The best place to read up on it is the Jenner & Block law firm's website. You can print it and read it or download it. In Illinois where they are, the plaintiff must establish four elements with either the TRO or the PI: 1) a protectible right; 2) irreparable harm; 3) an inadequate remedy at law; and 4) a likely success on the merits. They have the rights to the client lists, company secrets, phone lists, customer habits, etc, and any attempt to use the LLC's information will soon find the dissociated partner without the right to run his company if he doesn't apply scrupulous attention to maintaining his fiduciary duty to / with the LLC. His defense might try to suggest that the customer list was in his memory, but some judges (and you absolutely HAVE to check out the judge's history) automatically give the TRO and are extremely liberal about the PI too. That Likely Success on the Merits... has a very low threshold. And they can keep the partner on the hook by delaying discovery for at least two years... so be careful not to allow an open ended PI.

My advice is to take a month in between dissociation and starting the new company to get the work done... save up so you can afford it, but leave it all to AFTER dissociation. AND DON'T TELL YOUR CLIENTS ANYTHING.
http://www.jenner.com/files/tbl_s20P...tions_1.06.pdf

But this post seems to be coming from the original LLC. Seems you might need to have a heart-to-heart, hire a mediator to come in and help correct the damage being done by all the unfriendliness within the LLC. You need to address it before it kills the golden goose... because even if you get the TRO and PI, it will be limited to your current/recent customers and most businesses need the new clients to keep their doors open. It is often the new clients who spend the most money... and you really don't want to share them with the ex-partner 50-50 or worse. Having competition who is known in the community and in that kind of business will hamper your LLC greatly.

My advice to any partner dissociating is to work very hard to correct the dissention before you risk a PI. It is hard to stop a TRO, but a PI is where you need to do IMMEDIATE depositions and find out what they are bringing as a basis for their case. And don't let some dumb lawyer tell you it is NOT an evidentiary hearing... like happened to someone I know... because that will be your undoing. And if a PI is ordered, be ready to suggest an agreed list of untouchable clients for a certain time instead. Do NOT let yourself be blindsided by not reading a PI written by the LLC lawyers. IT MUST BE READ AND LIMITED TO WHAT WOULD BE FAIR. NO UNLIMITED TERM, NO UNLIMITED LISTS, ETC. No preventing you from working for someone who deals with those clients in another kind of business. THAT literally prevents you from accepting employment with ANY local firm.