Hello Idris,
you would possibly need a branch-step in the Workflow to see, if that's the last approval step or not. On the last one, create a step with the reject-processing option, where the other is missing this. Possibly you'd also need to separate task definition TS...... to be involved.
If there's an authorization involved, it will be an ACTVTY on S_WORKINGWI, but you'd restrict an involved user from rejecting /any/ step here, disregarding if that's the last approval or not.
Florin