U.S. banking regulators summoned Wall Street's biggest lenders to New York's Pierre hotel in November to hammer home a message that had gone largely unheeded for more than a year: Stop arranging risky corporate loans that were inflating another credit bubble, or potentially face fines or suspensions.

The warnings from the Federal Reserve and Office of the Comptroller of the Currency are starting to sink in.

Debt levels for companies funding takeovers in the leveraged-loan market fell in the fourth quarter for only the second time since 2012, according to Standard & Poor's Capital IQ. Banks including JPMorgan Chase & Co. are passing on risky debt deals again this year, most recently a $445 million loan for an acquisition by KKR & Co.'s Alliant Insurance Services, according to two people with knowledge of the deal. The crackdown may reduce loan issuance by as much as $80 billion in 2015, according to Bank of America Corp.