Is it possible to remove a track from the cloud ?
Not at the moment. You don’t need to worry about how much cloud storage space you are using though.
And if I change the tag of a track after it was uploded in the cloud, is it reflected in the cloud as well ?
No, cloud tracks are not updated after their initial upload.
Your tags are in the Lexicon database, so if you also keep a backup (in the cloud or on your computer) then you can always write tags again.