Hi,
It Depends on What Ribbon Button you want to Hide.
If the Ribbon button is some Default button in CRM then you can Easily Disable by Editing User's Security Role.
For Example If don't want to Display "Export to Excel", you can Hide the Button by just editing the that User Security role i.e., you can un-check the "Export to Excel" authority as shown below.
When user has the Access to "Export to Excel" button.

When you don't want to Display "Export to Excel" button to that User.

You can Work around or even may be design a new Security Role from the Existing one and manage user access to various functions and entities like this.
Hope you find it useful.
- Purush(MCTS) Rogrand Consulting(www.rogrand.com)China