Go to the storage container in question and click "Access" (1) and "User management" (2) to see who has access to your data. There you will see the access key type and duration of keys. You can revoke keys in this view (3), but note that this does not take effect until the key has expired, which is a good reason for choosing keys of short duration.
If the key is marked with "Not retrieved" (4), then it can be immediately revoked, as the user has not yet activated the key to access the data.
You can also see who has "Data Steward" access, which persons they have shared the data with (5), and you can remove the access if necessary (6).