Simulink: comment block by set_param in callback

I am trying to use the PostLoad Callback to comment out Simulink blocks depending on parameters. So, in a file I have a parameter

UseBlock = 1;% or 0

The Postload function runs this file and knows the value of this parameter and if UseBlock is zero, the Postload function is supposed to comment out a certain block, if the value is 1 it is supposed to uncomment the block. I am using

set_param('Model/Block','Commented','on') % or 'off'

However, it does not work and interestingly any code occuring after this line (in the Postload Callback) is not executed. But I do not see any errors whatsoever.

Any ideas?