The phrase "extremely forbidden" is correct and can be used in written English.
You can use it to describe something that is either unacceptable or prohibited in a very strict way. For example, you could say, "The use of cell phones in the library is extremely forbidden."
The Arctic presents an extremely forbidding environment, with sea ice, howling winds and stormy conditions that will make drilling difficult and any cleanup far more complicated than it was in the warm and relatively benign waters of the gulf.
They have faced extremely forbidding circumstances and learned to find a way to overcome them -- you can too.
An anecdote from the period recalls that his master, an illiterate and extremely frugal man, forbade Evans the use of candles to illuminate his reading in the evenings.
Some parents are extremely strict and may forbid you to email people again.
During basic military training (BMT), tobacco use is strictly forbidden in all service branches, but tobacco relapse (and initiation) following BMT is extremely high.
However, after some time (such as after the first year) the women could long for forbidden food (like cheese, bread, potatoes and wine) and thus found the new diet extremely difficult to keep.
The diocesan assembly has since forbidden priests to act without authorization as election observers, and has warned that it is "extremely dangerous" for clerics to violate the overall church rule against participating in election campaigns.
"In Saudi Arabia, where any whisper of dissent can land you in prison and women are forbidden from getting behind the wheel of a car, Charles may find that numerous topics are extremely sensitive ones".
According to Weeks, "[a]s a practical matter, it is extremely difficult to successfully prosecute a section 201 action if reference to a defendant's legislative acts is forbidden".
