I completely agree for the need for boundaries, but I think we've done a most excellent job of setting them!
The current boundary is speech that immediately incites violence, I believe the supreme court interpretation has been very generous towards the notion of free speech, including hate speech, as it should be!
Here's why:
We should let the radicals be racist and make radical statements as much as they want. At the point at which they turn violent or call for specific acts of violence we should step in and shut that stuff down.
A blanket militant action against the whole ideology will no doubt show better results on shutting down the movement on a short term.
But the notion of freedom of speech is not meant to protect these radicals, it is meant to protect ourselves and the integrity of society! We cannot make exceptions to these rules just because it's more convenient to us now, because this weakens the principle. This is the whole slippery slope argument.
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety."
If you wish to change the lines on which we use force, you must make a coherent case based on principle, not on convenience or anecdotes, as the principles themselves already account for, in a deep way, the history to which you allude.