A clamper charged a couple an extra £100 because they swore when he asked for the release fee.
Clamping company Bulldog Services, which operates in Bath, Somerset, demanded the fee on top of the £125 illegal parking bill.
The clamper said the £100 charge was enforced when extra staff had to be called because of abusive behaviour.
But the woman involved said her husband only swore at her after discovering they had been clamped.
The 33-year-old woman said: "My husband in no way swore at the attendant, he turned to me to say it.
"They are bullying people and I don't know how they are getting away with it."
The RAC Foundation deputy director Sheila Rainger said: "It is just another example of how clampers are fleecing the public."
Ms Rainer said there is a "huge black hole in the legislation and there needs to be an enforceable code of practice."
"Clampers must have an SIA (Security Industry Authority) licence but there's no regulation of fees," she added.
The Bulldog Services clamper, who would be named only as Karl, said signs in the Comet car park which he patrols clearly stated a £100 charge would be enforced if there was abusive behaviour and a "recovery unit" is called out.
Karl said he asked the man to read the sign four times and said he only makes £25 for each clamp he undertakes.
Karl said: "He aggravated himself. I explained it to him.
"I was more than polite to him. If I am not polite than I lose my job."