I googled about hiding buttons in MS CRM 4.0 and I was sent to this brilliant post from David Fronk from Dynamic Methods Inc.. Applying his script to the entity's form through the onLoad event I was able to hide permanently a button. However I was looking for more!
I wanted to hide a button only if a specific field was filled. And how did I pull this one off? Simple! Just added the piece of code to the field's onChange event! It's important to keep the code in the onLoad event if you want to keep the button hidden when you save and close your entity and then reopen it.