The customers posted a YouTube video about the Sunday incident in which the Papa John's delivery man mistakenly called the customers with his phone in his pocket, ABC News reported Tuesday.
The call went to voicemail where the driver can be heard complaining about the $5 tip he received and referring to the customers in derogatory terms.
"I guess that's the only requirement for being a [insert N-word] in Sanford," the driver said. "Yeah, they give me five bucks there -- fine outstanding African-American gentleman of the community."
Schnatter issued a public apology about the rant on the pizza chain's Facebook page Monday.
"Friends, I am extremely concerned to learn about the reprehensible language used by two former employees in one of our restaurants," the post said. "Their thinking and actions defy both my personal and the company's values, and everything for which this company stands."
Those "responsible for this absolutely unacceptable behavior were immediately terminated," he added.
Teenager Trayvon Martin was killed by George Zimmerman last year in Sanford, Fla., an incident that has fueled racial tensions in the town.