By raiding their headquarters. The Star-Trib says five people were arrested, though 100 others were handcuffed and questioned before being sent on their merry way. Snip:
In a statement Saturday morning, Ramsey County Sheriff Bob Fletcher said the St. Paul raid targeted the RNC Welcoming Committee, a group he described as "a criminal enterprise made up of 35 self-described anarchists...intent on committing criminal acts before and during the Republican National Convention."
"These acts include tactics to blockade and disable delegate buses, breaching venue security and injuring police officers," Fletcher said. Deputies seized a variety of items that they believed were tools of civil disobedience: a gas mask, bolt cutters, axes, slingshots, homemade "caltrops" for disabling buses, even buckets of urine.
I'm a little curious how authorities knew about their plans before they were actually carried out.