Hide Cividiscount box from specific contribution page
There are a few options to make it so only Admins can apply discounts.


Create a price set for memberships and add a text/numeric quantity field called 'Discount', set the value to $-1.00 and set the visibility to 'Admin'. With this option you can apply custom discounts when entering memberships via the admin panel without using a CiviDiscount Code. The special field for discounts will only be visible to logged in admins. 
If you are comfortable with some coding you can use jQuery to hide the CiviDiscount form section from the contribution page template via a custom template. Keep in mind this will hide the CiviDiscount form from all contribution pages. 


Hope this helps