There is no docker untag command, so, the answer is find the repository which using the image and execute
docker rmi <REPOSITORY>:<TAG>
There are the commands we executed before to tag a new image with an image id
docker tag <image id> hkccr.ccs.tencentyun.com/xxx/fff:v0.1.4
And remove the tagged image
docker rmi hkccr.ccs.tencentyun.com/xxx/fff:v0.1.4