Hi Dara,
I assume the issue is that you are not able to delete the task at project/workspace level as the task is coming from the workspace template. If so, please check if you have template creator group assigned to yourself and then you can got Manage Templates -> Create new version -> delete the signature task -> create a notification task > Publish the template changes. The changes will be applicable to all new workspaces/projects you create from this published template. The changes can also take effect on existing project/contract workspaces but the functionality is controlled by a parameter and also the status of your project/contract workspace.
In case you are stuck with a already created workspace(s) then it wouls be a good idea to raise a service request with Ariba support team so that they can help you with parameter changes required if any and let you move forward.
Regards
Vilas