Under the First Amendment, you may no more compel speech than prohibit it.
In the 70's SCOTUS took up just such a case.A guy in New Hampshire was imprisoned for deliberately obscuring the words "Live Free Or Die" which have been featured on the state's license plates for decades.He was some kind of Marxist pacifist.
SCOTUS ruled in his favor stating that the 1st Amendment also allows you to remain silent and thus the state's requirement that those words remain visible was a violation of his 1st Amendment rights.
This is a good thing because I've deliberated obscured the words "The Spirit Of America" which appear on my state's plates (my state absolutely *not* being as claimed on the plates).