Sorry- system glitch. Here it is again:
There two types of issues here: Copyright and Trademark.
If the licensing agreement did not define the license as something more like a work for hire (in which the copyright transferred to the company buying the rights), but simply a right to use the material, then any copying or continued use could violate copyright law.
Copyright is automatic upon creation and "fixation" in a tangible form (once drawn on paper or digital file, that's a tangible form). But to enforce that copyright in court it is necessary to first register the work. Here's a link to the Copyright Office to do so.
A drawing is classified as a "visual work" for registration.
I might have a lawyer (or if lawyer's help is not in the budget, do it myself), send a certified "Cease and Desist Letter" to the company, demanding that it stop copying and using the material, and reminding it that copyright infringement can result in thousands of dollars in court-ordered restitution for each item copied, each time it is copied. That sometimes works.
Send the letter by certified mail, return receipt requested (keeping copies of both the letter and the return receipt as evidence).
The trademark piece is trickier -- and not something to approach without expertise (as a lawyer might have). Trademark is a kind of consumer protection law that businesses use to ensure that others don't steal their good name or those things that stand in for it. Thus, MacDonald's has a lock on double golden arches when used in a restaurant logo, and even for other things that consumers might assume is provided by MacDonald's Corp.. Similar arches, that are not identical, are not okay under this type of the legal theory.
The thing is, it is necessary to provide evidence that consumers associate a certain look or feel with a particular original producer of something (whether artwork or widgets). And that is the tricky part that may require a lawyer's help. While trademark registration is not required, it is helpful. I'd make this issue part of any discussion with own trademark lawyer who, alone, can provide confidential information protected by attorney-client privilege -- something that is simply impossible (for obvious reasons) on a publicly-accessible legal information web site.
Please let me know if anything needs clarification by typing a response in the next text box.
I wish you every success with your work!