How to add a data limit to free trial hotspot users??

HI

who can help write a scripts, limit hotspot free trial user data usage in 50MB.
i use mikrotik 6.15 RB951Ui-2HnD & Huawei E367 3g modem for my hotspot system.

Regards,

have Just added a Script

http://forum.mikrotik.com/t/hotspot-data-limits/90475/1

this works well for me