Go to > Control Panel > System Admin > User Listing > Edit a user > go to "Optional" Tab > on Time Curfew field, you can set the times when that user cannot login.
Time 
Accepted time frame format (* means all number):
...
for each settings, you need to enter new line, so if you have 2 settings, you will have 2 lines of settings
Example 2:
User Cannot login from 11pm to 7.30am, the settings will be :
* 30-59 23 * * ?
* * 0-7 * * ?
* 0-30 7 * * ?
