I've started a full discovery scan with all file types, extensions, actions is monitor and tag those files. Is there anyway I can halt the scanning process?
Alternatively if there is a way to untag those files, do let me know too.
Why would you do that?
To stop the scan create a new Agent Configuration with Discovery unchecked in the Miscellaneous section and apply it only to the affected machine.
Remove the tag from the DLPe policy and apply to ePO. This will at least ensure the old tags are not triggered by any rule and/or reported.