If it is a dedicated/VPS customer with their own server you can create a dedicated plan which will enable you to select the server at the time of subscription. All their sites under this subscription will be placed on their dedicated server and no other customers will be placed there.
Shared packages aren't tied to a server, their websites are placed according to placement rules under settings->platform. If you want to limit provisioning for any customer on this plan to a specific group of servers you can create a server group and specify that server group under the plan server group setting.
