Kirkendoll said his campaign did not know the contribution was prohibited when it was made.
"We thought it was perfectly fine to do it, or else I never would have done it," he told the newspaper.
He said the Secretary of State's Office has cleared him of any wrongdoing.
Leach said in his letter that the office is troubled that the receipt of a prohibited contribution is not addressed by state law or regulations. He commended the PAC for voluntarily refunding the donations.
"Federal regulation apparently covers this and requires repayment once the prohibited status is brought to the attention of the recipient," he wrote.
Leach said the Secretary of State's Office plans to study the federal model. The Legislature would have to amend the law to add any circumstances where a PAC could not receive money.
The existing law prohibits PACs' receipt of anonymous donations, cash donations that exceed $50, donations from other PACs, and more than $1,000 from the same contributor during an election period.
However, a PAC that makes only independent expenditures can receive more than $1,000 from the same contributor, Leach said, citing a federal court ruling.