Saying no to student requests for excessive appointments and help solving homework
I would limit the amount of time you dedicate to a single student to a maximum of 20 minutes a week. This is a reasonable amount of time to discuss the main problems a student has. This does not mean that you cannot make exceptions, but it is good to have a general rule.

As a side remark: You will not make everybody happy. If somebody wants an appointment and you say "no", and he insists, you can still say "no" and walk away.