After you've approved the model, you cannot delete stages when editing a draft. Instead, you can merge that stage with another.
1. Click Marketo Home and select Analytics.
2. Click your approved model.
3. Click Edit Draft.
4. Right click the stage you want to merge and select Merge Stage in the menu.
5. Click the specific stage in the pull down.
6. You can re-approve the model by selecting Approve Model Draft in the Model Actions menu.
Pick None in the Merge Stage pull down to remove the leads from your model