While testing a workflow we discovered that the person requesting a workflow approval can delegate the workflow to themself then they are able to approve their own request. Is there a way to disallow workflow approvals by the submitter/requestor?
create a validation rule in task form that checks whether current user/approver is the same as one who created item or started workflow (or possibly both), which suits you more.
this has to be implemented per every single task (form).
there is no possibility to prevent globally that an initiator gets assigned his own task to approve. simply because delegation doesn't take into account any application rules/restrictions on who can be approver and allows to delegate task to any active user (account).