Pepper spray. And promise Moveon.org that when you are President, you will prosecute them and their donors under RICO.
Hold the pepper and add some lead.
Spray them with soap (non-biodegradable) and water. That would be worse.
No, don't do that. Don't warn them. Just gather the evidence quietly, and then lower the boom on them. Maxixmum sentences, everybody goes to high-security prisons, everybody does hard time, federal time -- no namby-pamb "good time", no paroles, no early releases. Hard, hard, hard-rock time for trying to kill the Republic.
The attorney general admitted recently to investigating whether “climate deniers” can be prosecuted under RICO.