Its impossible to ban extremism in political parties, or in politics in general, because to do so would be an attempt to legislate and ban certain thoughts. Its neither beneficial to society nor technically feasible to ban thoughts or feelings, and any attempt to do so would simply be a long term waste of time.
By banning political parties, you will simply drive them underground. You will also make them into political martyrs, drawing the sympathy of the population. Finally, you will give them lots of publicity that they otherwise would not have. Extremists parties are often quite small in popularity and trying to ban them only gives them the publicity that they otherwise could not get.