If you are not using a virtual machine but may still need it, you can stop it. While in the stopped state, you will not incur any utilization costs.
From the details page for your virtual machine, click on "Stop Virtual Machine" on the far right panel.
Or, from your browser view, click on to stop your VM. You will be asked to confirm this action.
Note: Although you do not get charged for virtual machines while in a stopped state, there may still be some charges for the storage these VMs occupy.