Is there an instance where you would charge someone with conspiracy?
This is another issue. The question at hand is whether or not advocating an idea can be made illegal. Expressing one's opinion regardless of the nature of that opinion is a very different matter than making plans and preparation toward an illegal act.
Identify those doing the swinging and charge appropriately. The remainder will get the idea that listening to this guy isn't good for your health.
In a court. With a jury.
How many people are he allowed to conspire to commit crimes with before you go after him as well?
As much abuse as I have observed in conspiracy charges which range from actual proven planning and organizing of an intended crime to mere expression of an opinion (i.e., [insert crooked politician of your choice] should be hanging from a yardarm for treason), to merely stating that corporal punishment is a necessary and workable solution (as opposed to participating in working over children with dangerous blunt instruments) in conjunction with the tools who are on most juries because the lawyers won't allow anyone who isn't a tool, you will find little confidence here that a modern jury trial carries any more weight than a self-selected 'jury' on a gradeschool playground. In fact, I would probably bet on the kids being more fair and reasonable.