SoftLayer SLA says that you have to cancel the device 24 hours prior to the next billing cycle avoid the billing for the next month. Follow the below steps to cancel the device.
Step 1 : Log in to https://control.softlayer.com by entering your username and password
Step 2: Go to Devices -> Device List
Step 3: You will see all the devices which you have purchased. Go to the device which you want to cancel. On the right hand side of the device name, click on Actions
Step 4: Click on Cancel Device, as shown below.
A cancellation ticket will be raised and your device will be cancelled in 24 hours. Please ensure to take the backup of the data. Once your device is deleted you will not be able to recover your data in any possible ways.