A college professor who suggested that President Donald Trump needs to hang to "save American democracy" won't be seeing the inside of a classroom anytime soon.

Fresno State professor Lars Maischak was punished by the university for his threatening statement via Twitter, which radio talk show host Charles Butler says is a lesson for Trump haters to learn.

"That is not a part of free speech from what I understand," Butler says. "It will be taken very seriously."

And it was taken seriously. Fresno State must complete a three-year contract with Maischak but the professor has been assigned to the job of designing online college courses until the contract runs out next May.

It's nothing new for left-wing professors to rant about Trump and Republicans, such as a Florida professor who stated - also on Twitter - that Texans deserved Hurricane Harvey for supporting Trump's presidential candidacy.

The University of Tampa cut ties with sociology professor Kenneth Storey after he made that statement.

At the same time professors are threatening President Trump, says Butler, American citizens are being punished for their support of the president.

"We need to pay attention to the kind of language that's being used out here against not only Donald Trump but against his supporters," he says.