Authorizations to create substitutes

[Hassine, Felix]

Hi everyone,
 
We have a workflow related to approval of invoices with monetary limits
assigned to different agents.
Unfortunately, some agents may conturn our internal rules  by assigning
their own substitutes (who may not have this level of approval...). If a
user can assign up to 10000 USD or invoices, assigning user X to his
substitute authorizes this user X to execute the approval task (Based on a
DECISION step...).
 
Is there any way to restrict users from maintaining their own substitutes?
 
I would like to avoid re-coding the DECISION step in order to verify the
monetary limit of the approver. Has anyone encountered such situation?
 
Thanks for any info.
 
Best regards,
 
Felix