This is a genuinely complex situation because it involves the intersection of trademark law, partnership law, and business dissolution. Let me walk through the key issues:
1. Who owned the name? When a partnership uses a trade name, the name is generally an asset of the partnership, not of any individual partner. Under the Uniform Partnership Act (UPA) (adopted in some form by most states), partnership property belongs to the partnership itself. The fact that you personally came up with the name doesn't automatically make it your personal property if it was used as the partnership's trade name.
2. What happened at dissolution? When a partnership dissolves, its assets must be distributed. You said you split assets but didn't have a formal dissolution agreement. If the trade name wasn't specifically assigned to either party, it creates ambiguity. Neither partner can unilaterally claim it without the other's consent.
3. Trademark rights: Was "BrightSpark Creative" ever registered as a trademark? If not, you may still have common law trademark rights based on use in commerce. Common law rights belong to the first user of the mark in connection with the specific goods or services. But here, the first user was the partnership, not either individual partner.
The strongest argument is that Sam cannot unilaterally appropriate a partnership asset. By registering the LLC under the partnership's name without your consent, he's arguably converting partnership property. You may be able to seek an injunction requiring him to stop using the name.