I assume you're running the firmware supplied with the box. The trick is actually pretty simple: A text file on the LS keeps status information on the backup jobs. In your case, the status is listed as "run" or something like that. You will have to change that to "done". That's it.
First a few warnings: This is probably voiding your Linkstation warranty. If done incorrectly you might break something, that isn't easily fixed. Also Georgs handy little tool, acp_commander, is provided as-is. I don't know how well it has been tested, if at all. It worked for me, though. I know there is a newer firmware for the Linkstations, than the version I tried this on. I was running 1.03 as far as I can recall. I dont know if this will work on newer versions. Furthermore, the following is based on this thread, the wiki and, more importantly, my recollection of events passed. My recollection comes with ABSOLUTELY no warranty
You will need:
1) to know your way round a linux terminal via telnet.
2) to be able to use a text editor - don't know if original firmware comes with other than vi - be careful! - it's tricky if you're not used to it.
3) Java on your PC (standard runtime - not sure which version - try the latest).
4) to disable any firewalls between you and the linkstation (including the one on your own PC). Or at least make sure UDP port 22936 is accesible on the LS.
1) Get acp_commander.jar from http://buffalo.nas-central.org/download/TOOLS/ALL_LS_KB_ARM9/ACP_COMMANDER/
2) Read the README.txt in the same place !!!
3) Execute from your PC's command prompt: java -jar acp_commander.jar -t <your_linkstation_ip_here> -o
4) Open a telnet connection from your PC to the Linkstation, ie. "telnet <your_linkstation_ip_here>". Username is root, no password.
4) Find the file that corresponds to the hung backup job in the folder /etc/melco/. As far as I can remember they are named backup1, backup2 etc., each corresponding to the jobs in the webinterface.
5) Open the file in a text editor and locate the line stating the status of the backup job. It will say something like "status=run" or similar.
6) Now change that to "status=done", save the file and exit the editor. Also exit the terminal.
7) Very important: The above steps have enabled telnet access with a blank root password. Securitywise this is a crime. Rebooting your Linkstation now is a really good idea, since this will close the telnet access and reenable the root password.
8) Check the webinterface - the job should be configurable again.
You might want to read some of this: http://buffalo.nas-central.org/index.php/LS-GL_Custom_Firmware_Development#Georg.27s_Method