Before you can lock a user to a warehouse, go the Admin panel and enable the Warehouses Restricted by Sales Rep? setting under the Warehousing / Order Entry section.
If items lack an assigned warehouse, reps will be unable to see the item.
How to lock a user to a certain warehouse
Go to My Account
Click Manage
Scroll down to the bottom under Members
Select the name of the admin you want to lock the warehouse
In the Accessible Warehouses area, click the checkboxes next to the warehouse you want available
Select Save
Please note that you can also accomplish this for users who are admins; however, they would be able to change this setting themselves if desired.
How to set the default warehouse for an individual user
Go to My Account
Click Manage
Scroll down to the bottom under Members
Select the name of the admin you want to lock the warehouse
Select the Primary Warehouse from the dropdown
Select Save