Each class has a Register button. You can disable the button, edit the label, and turn it into an unclickable call to action.
From the left navigation menu, click Classes > Class Admin.
Click Classes.
Select the desired class(es).
Click Edit > Multi-Edit Basic Info.
Set Call to Action Override to YES.
Enter the custom text to appear instead of the button.
You may optionally click the color box to change the button color.
Click Save.