This is a common request in previous versions of AX. Usually you would want to stop the submitter from approving their own workflow.
Dynamics AX 2012 CU8 has introduced a new flat on the Workflow parameters. “Disallow approval by submitter”. This is a global setting and not workflow specific.
To test this out I created a user group with 2 users. One of them being my account.
I was allowed to submit it but I found the workflow history had an error. ie Submitter cannot be the approver.
I understand why Microsoft has done it this way but I am not a big fan of it. I don’t like introducing errors in the workflow history. It makes the workflow configuration very difficult. Lets say you had a team of 5 users. Each one was able to submit but not approve their own journals. Then you would have to create a complex workflow configuration to deal with all the combination.
I have done similar solutions in the past where the check was done at the time of approval. The user received an error on click of Approve.