Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If you are sure that they are going to Haram places then you should not take them in your taxi. Allah Says (interpretation of meaning): {…Help you one another in AlBirr and AtTaqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. …}[5:2]
For more details, please read the Fatwa: 81174 and 81389.
If you are not aware of where they are going then you may serve them.
Allah knows best.