If you're admin in plesk (for example as service-provider in your case) you can limit the recources for resellers, wich they can share to thier plans.
Login to plesk as admin.
Resellers -> Click on the specific Reseller
On the right side: Resources -> service package -> adjust/customize
There you can set the recources and permissens for the reseller.
If you have any further questions, let me know.