Making buttons not clickable once they have been clicked

Hi all, thank you very much for your quick responses.

I have tried the "transparent button" approach suggested by Judy and it works perfect!  Very easy to implement too!  Thanks a lot Judy.  

Nedim, great example! that reallly opened my mind on different SL possibilities!

Walt, Tom:
I have tried using states but it didn´t work. The reason for this is because I am using different buttons states to let the learner know what option they selected.   E.g.  I have added 4 different states to each of the buttons (Option 1, Option 2 etc..).  So whatever the learners select I have a set state trigger for the button to change to what´s selected. 

This means I can´t use any other states or I loose the above funcionality.

Here is what I mean:

Anyways, the transparent button was a great easy solution!

Thanks you all very much, you have saved my day!