Currently there is no GUI to remove saved logins from the drop down list on the login screen. The only way is to edit spark.properties file and guess which password<code> line correlates with which login. Maybe it should provide a right click context menu in this link with an option to Remove an entry. Or maybe it should delete while hovering an entry and pressing Del key on a keyboard.
Update: deleting a password entry in the spark.properties file deletes only a password, but a login entry still remains in the list (just with an empty password field, when selected). To remove an entry from that list, one has to go to Spark\user\ folder and delete the corresponding folder (by this deleting also the conversation history of this account, which could be undesirable).
So, maybe there could be two options for the entry removal (e.g. when right clicking an entry): Hide login (only removes it from this list, e.g. adding a "don't show" file or something into corresponding account's folder or tagging this account appropriately in spark.properties file); Remove account (which in turn shows a warning about deleting a conversation history of this account). When an account is hidden, it should still show up again when a user logins with it again.