how to delete the messages that remain on JotForm site ?

Example I receved n. 5 messages through one form ; how i may delete they ? (excuse me for the english..)

You can delete the submissions from the submissions view. 

http://www.jotform.com/help/269-How-to-view-Submissions 

You can delete individual submission by clicking on X mark on each submission. If you want to delete multiple submissions, you can hold Ctrl key on the keyboard and select multiple submissions. By clicking the button Delete all Submissions would delete all the submissions. Please be noted that once the submissions cannot be recovered in anyway once they are deleted from JotForm. You'll need to make sure that you have a backup of submissions. You may integrate Google drive, Dropbox, Google Spreadsheet, etc apps as a backup.

Hope this information helps!