There are several options. You can purchase more disk space, or in order to free some disk space you can empty the trash bin, keep less file versions, or remove files or folders from the backup.
1) Empty the Trash bin
Files in Trash bin are calculated in disk space usage. First you should check if the program's Trash bin contains any files. You can check this in client's Restore window that can be accessed from Backup page. Contents of Trash bin are displayed in Deleted folder in directory tree.
If Deleted folder contains any files, and if you no longer need to keep those files, you should empty the Trash bin. You can do this by clicking Empty trash link at the bottom left corner of the window.
2) Remove old file versions
You can also remove older versions of backed up files. Go to client program's Settings, then select Settings window. Change value for Store [x] versions on server. If you, for example, change it from 5 to 2, then only two versions of each file will be kept in the backup instead of five.
Please note that changing value for "Store x versions" does not delete versions and release space immediately, but old versions of a file are deleted only after a new version has been backed up.
3) Remove files or folders
Go to Settings, then select Files window. Remove some folders or files from backup by unticking the respective boxes. They will then appear in Deleted folder. Empty the trash bin.