Yes, they are allowed to say whatever they want until their death. After that, in the Judgement Day, Allah will give their punishment for their sins. You are basically not authorized to attack them. But, you should defend your religion, without exceeding the limit. What is the limit? It is what they do.
If they attack with their tongue, you should defend with your tongue, with arguments and proofs.
If they attack with pen, you should defend with pen, with verses, hadiths and academical writings.
If they are mocking with religion, that means they are trying to get support of third people, they are playing to the tribune; you should play to the tribune too and get their support. Most of the people don't like unrespectful people and you should get their support. If anyone doesn't laughes, then nobody makes a joke.
If they are attacking physically, first you should call help from authorities like police. If they don't do their job, you should go to the upper authority like courts, or media, or international community. If any authority doesn't do its job, this is the point you can take initiative and defend yourself.