Navigate to Components -> Fabrik -> Elements
Then select the element you want to hide inside your Form
On the left hand side, click on JavaScript -> Add
Published: Yes
Event: Load
Use the Predefined options
If this element: Shown
Action: ReadOnly or Hide
Element: Select the element you want to hide
..With Fabrik there is usually more than dozens of ways to achieve what you want to do. This method of hiding an element or making it read only is one way to achieve what you want. Keep this in mind going forward, that there are many ways to achieve what you want in Fabrik, if one doesn't work out then there are alternatives.
Hope this helps dude.