The answer is simple, revoke their carriage license. If they want to be a mullah, let them open a mosque, if they want to be a cab driver follow the city rules.
So a pizza delivery guy can not refuse delivery to a dangerous area because he will be DEAD, but, a muslim cab driver can refuse service to someone carrying alcohol.
How have we gotten to this strange time in our history?
Let Muslim drivers refuse to transport obviously carried alcohol. The free expression of religion is protected, and the government should not interfere.
However, the cab companies should be able to fire those drivers if their religious views interfere with them performing their jobs. The government should not be able to force employers to allow drivers to perform their jobs as they choose. Doing so interferes with the employers free expression of their own religion, and forces them to financially subsidize the Muslim drivers' religious views.
That is something the government should not be allowed to do.
If the drivers actions offend customers, they can express their displeasure to the company, and refuse to patronize that cab company in the future. The government doesn't need to get involved and should not get involved.
It's the government stepping in and forcing us to accept the behavior of others that is the real problem lurking behind this issue.
Removing the government from an issue as much as possible and the issue becomes much more simple, and can usually simplify the issue greatly and allow the people involved to solve it on their own.