Yes, most definitely.
Symbols can be as dangerous as actual weapons. Perhaps even more.
Back in Nazi Germany, the Blutfahne, or Blood Flag, was an extremely important symbol for the Nazi party. It was shown often and was used as a propaganda tool in a most efficient manner. It boosted the morale of those who believed in the Nazi regime and everything it cautioned.
Banning a symbol such as the ISIS flag is also symbolic in nature. It means that we will not tolerate those who honor the acts of barbarians. Those who carry the ISIS flags are murderers, rapists, torturers. They are committing atrocities as we speak. If you have the ISIS flag or if you glorify it, then you are approving of the acts perpetrated by these sick individuals.