A business partner and I were making mobile apps and i decided to step away to do my own thing. We never had a formal partnership so when i asked to be bought out or have a percentage of the income, he blew me off. Now the company is using my artwork(logos, app icons, interface designs, ect) without my permission. Since we never had a formal agreement and i never gave the company permission to use my artwork, do they have a right to use the artwork? Would a cease and desist letter hold in court if needed?
The answer to both of your questions is, "It depends."
What the answer depends on is the type of business you and your partner had. Even without a formal partnership, if you were planning to "buy out" his percentage, there was obviously some sort of arrangement.
Several factors are looked at in determining whether the works created can be viewed as "works made for hire" and belong to the company, or whether you have retained all rights in what you created while working for the company.
In other words, it is possible the artwork you created for the "company" could be considered company property and it is also possible that the company would be infringing on your copyrights by using your work, absent any agreement signed by you transferring the rights to the company.
You can always send a cease and desist letter. These tend to be more effective when coming from an attorney. You could inform the company that they are using your copyrighted works without your permission and are, therefore, infringing on your rights in the work. But it is important to know that a C&D has no force of law behind it. The company can toss it aside. So, before sending a cease and desist letter, you would want to know in advance if you have any legitimate legal action available to back up the C&D.
I suggest you have an IP attorney in your area review how the company is set up and how you operated within the business, to see if the company can legally claim rights to the work or whether you can legitimately claim all rights to the work you created.
Good luck.
One more suggestion: In the future, always always always have everything in writing so that disputes such as this are less likely to arise.