To remove the data from your SwiftKey Cloud, then you can do so by going into SwiftKey Settings->SwiftKey Cloud->Scroll down to ‘Delete cloud account’ and select ‘OK’. Please note that this will unregister all of your connected devices, and also delete your backed-up data from SwiftKey Cloud.
Local data stored on device:
Go into SwiftKey Settings->Advanced->Clear data on this device.
See the how-to video here.