Under Provisioning > select SIMs > enter ICCID > click search then click the ICCID in the Results.
In the Overview tab, under the General Section on the left, look for SIM Status, then click on the cog wheel icon. A pop-up window will come up asking to confirm this action. Type the word TERMINATE to confirm, then select TERMINATE SIM.
Terminating a SIM will effectively disable all SIM services, and you will cease having access to all controls for this SIM. This action is irrevocable. Please note, it may take up to 20 minutes for changes to be reflected.