Sadly I run in the same situation for another similar request. I have the idea that one has to update the permission set for the users to stop them from detaching an approval step. I've not checked yet what that function would be to remove/disable from permission set.

Thank you for the response , I tried to uncheck the routing template node in the permission sets but I still can detach it 

Do you perhaps have full access to DocMan?

Did you check via an user that has only limited access to other parts of IFS as well?

It finaly worked with unchecking APPROVAL_ROUTING_API.Remove__

@Hanane Thanks for sharing.