What can I do if I have proof that my idea was stolen? How?

These so-called "invention companies" have a horrible reputation across the country. If you can prove that you had the idea before anyone else, that you submitted the idea to this company because it told you in writing that it would help you patent or market your idea, and the company used your original idea to create a product and market it without abiding by the terms of that written agreement, you may be able to prevail in court. I assume you have already sent the company a "cease and desist" letter. Next, contact your attorney general and the attorney general in the state the company is located in and file complaints with both. You should also talk to an IP attorney about filing suit in court.

Hire a lawyer. Bring all evidence you have regarding working with the invention company - correspondence, canceled checks, everything.