For instance, Findlaw defines "assault/battery" this way:
Assault/Battery In most states, an assault/battery is committed when one person 1) tries to or does physically strike another, or 2) acts in a threatening manner to put another in fear of immediate harm. Many states declare that a more serious or "aggravated" assault/battery occurs when one 1) tries to or does cause severe injury to another, or 2) causes injury through use of a deadly weapon. Historically, laws treated the threat of physical injury as "assault", and the completed act of physical contact or offensive touching as "battery," but many states no longer differentiate between the two.
So anyone who publicly states that he is going to "behead the person who insulted the Prophet" should be taken seriously and locked up for assault.
In the US we are not (yet?) in the position of the Europeans where 12 and 13 year old girls are kidnapped and sent back to Pakistan to be married to their cousins. But our laws would (and should) prevent this from ever happening here.
Same thing with schooling females. And spousal abuse laws should apply whatever the religion of the person doing it.
We have the laws in place, but judging by the Europeans, it is the enforcement of the laws that is in question.
"So anyone who publicly states that he is going to "behead the person who insulted the Prophet" should be taken seriously and locked up for assault."
Yup. Enforce the laws. Prosecute people who break them.