First, how many more French people are going to be brutally murdered by Islamicized death-cult fanatics before the French government actually starts fighting back?
Second, how are they to fight back?
In answer to the second question there are a number of easily imagined options. The French government could begin a policy of deporting those Islamist death-cult preachers that aren't citizens; they could seize funds from mosques attended by fanatics and distribute these to the victims of Islamist attacks.
Then bulldoze those mosques, raizing them to the ground.
In cases where the perpetrators hold French / EU citizenship, the French government could choose to exile fanatics and death-cult preachers to some remote island in the South Atlantic. Without impinging on freedom of speech, the French government could amend or abolish anti-discrimination legislation to allow French people to freely discriminate against and ostracize Islamists, thereby providing an incentive for reasonable Muslims to disassociate from Islamist nutters. There should also be public programs to encourage apostasy and to protect apostates from violent reprisals.
These are all relatively small, practical measures that the French government could take (but probably won't). They may not be sufficient to stop Islamist attacks occuring. Personally I would add the legalization of personal firearms ownership, though I suspect this may be culturally "impossible" for the French.
There is also the nuclear option, and I think resistance to the idea of nuclear weapons being used again is going to be increasingly eroded as further Islamist attacks occur on western soil and the public sees their elected governing class not doing anything much about it. And it may only be a question of time before one of these Islamist death-cult groups acquires or "appropriates" nuclear weapons themselves.