To get a list of ftp commands type HELP.
"prompt" or "prompt off" should turn prompting off (yes/no to delete
"mdelete *.jpg" will delete all jpgs in the current directory. If
prompting is on (usually the default) you will be asked Y/N for each
file before the delete is done.
The m* commands (mdelete, mget, mput) are the mass delete/get/put