With this PrestaShop module you can define minimal purchase value based on groups of customers. Each group can have own unique value of minimal purchase. When someone will try to order less stuf than value you defined - it will not be possible to continue order process. PrestaShop will inform customer about that (message about minimal purchase value).
more informations about this module you can find on official module website
Minimum purchase value by customer group
You can find there only valuable solutions for your e-commerce shop based on PrestaShop engine. Search the best plugin for your store that will meet your requirements and just use it.