How to stop a running backup?
Be careful: There is no way to cancel a running backup from the client.
If you stop or kill an application that started a backup of Firebird database, the backup will keep running on the server and consume valuable system resources. The only way is to kill the Firebird process on the server.
If you use Classic, it's rather easy and harmless, you just have to detect the correct process. You can do this, by finding a process that holds the backup file open. On Linux it's easily achievable with:
lsof | grep backup.gbk
On Windows, you need to use some utility to list the open files and find the process ID. As backup is read-only operation, you can safely stop that process.
If you run SuperServer, the only way is to restart the entire Firebird server. One more + for using Classic over Super Server.