how to prevent cyber crimes in your hostel?

[adaviel]

No, you can't prevent cyber crime - which could be just posting hate mail on Facebook. Any more than you can prevent someone staying in the hostel from robbing a gas station or phoning in bomb threats.

She could get guests to sign that they agree to an AUP that says don't break the law or interfere with other guest's usage of the network, and that the network may be monitored for diagnostic reasons. Then maybe actually monitor it at the router - log MAC address, dates and times.
In the unlikely event that the police do want to investigate something, show them the logs and the guest register.

Most hotels don't seem to bother - just give out a password to any guest that asks, or have an open network.