Hi
The best way would be to have the creator/artist sign a waiver and transfer of rights. This is an agreement between you and them (it's about 1.5 - 3 pages long) in which they clearly state that they are transferring all the rights to you and waiving any future claims.
I advise having a lawyer draft this for you* (yes, I also draft these documents), or if you don't have any money, I am sure you can find some free or cheap templates online from one of the website that provide legal documents.
* the reason I advise using a lawyer/attorney is because this type of document isn't too expensive, and you need it to be tailored to your specific needs.

Good luck
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