Hello Achraf,
When viewing the Mail Queue, you can click 'All' to view all of the messages in the queue on one page, click the top radio button and click 'Remove'. This will clear all of the messages including any legitimate ones in the queue. Alternatively, determine if you are using Qmail or Postfix and then Google around for the relevant removal tool with the necessary flags to match a pattern. If all of the messages are sent from the same email address, have the same subject and/or have a common phrase in the body of the email, you will then be able to use the command line based tool to purge the queue based on the common pattern. I hope this makes sense. Good luck!
Regards,
MrKman