A liberal state protects individual rights. Theft, fraud or attacks against one's body or life are punishable. After providing one with a fair trial one can be convicted of such crimes. Members of “Golden Dawn” have been, among others, accused of crimes against property or life and thus being convicted of them (if evidence occurs) will be just. Being democratically elected does not affect one's capacity to be found guilty.
For example, it is not punishable itself to be the leader of a Nazi party, but it is punishable for the leader to ask the party's chief members to attack or kill another specific individual.
Democracy must deal with Nazis not by depriving them of their individual rights. Nazis must, as individuals, have the right to use hate speech, recruit people, converge or hope to the substitution of the constitution by their misanthropic “moral code”.
Democracy, on the other hand, can use only just weapons to fight them. Justice (quick and functional), freedom of expression and knowledge can isolate “Golden Dawn” from the liberal society, in which it does not belong.