I could not delete any print jobs although I was signed in as an admin. Luckily I found an article from Microsoft which describes in which directory the spooler stores the print jobs. Per default the print jobs are stored in

%SystemRoot%\SYSTEM32\SPOOL\PRINTERS

Now I just had to

net stop spooler

Go to the default directory and delete all files, then

net start spooler

Voila, the queue is empty.