But then, of course, you need some kind of enforcement. And once you set that up, it turns out that it is subject to the same problems as other law enforcement / intelligence agencies:

"In 2001, the federal government, the Bundestag, and the Bundesrat jointly attempted to have the Federal Constitutional Court of Germany ban the NPD. The court, the highest court in Germany, has the exclusive power to ban parties if they are found to be "anti-constitutional" through the Basic Law for the Federal Republic of Germany. However, the petition was rejected in 2003 after it was discovered that a number of the NPD's inner circle, including as many as 30 of its top 200 leaders were undercover agents or informants of the German secret services, like the federal Bundesamt für Verfassungsschutz. They include a former deputy chairman of the party and author of an anti-Semitic tract that formed a central part of the government's case. Since the secret services were unwilling to fully disclose their agents' identities and activities, the court found it impossible to decide which moves by the party were based on genuine party decisions and which were controlled by the secret services in an attempt to further the ban. The court determined that so many of the party's actions were influenced by the government that the resulting "lack of clarity" made it impossible to defend a ban. "The presence of the state at the leadership level makes influence on its aims and activities unavoidable," it concluded."