Do they still need to be able to use the computer? If not you could use the parental controls to disable the account during the hours you don't want them to have internet access.
You could also limit the entire pc through your router during certain hours, but that would disable inet for the pc not just for the account.
Can you clarify what you mean by restrict internet particular user account? Does the account never need to have inet access, or you just want to limit the bandwidth it uses?