Yes the FTP accounts can be safely deleted without deleting the addon domain head over to FTP accounts with Cpanel (As always though its always good practice to do a site backup just in case).
The FTP accounts even though created when creating an addon domain work independent. Personally I wouldn't bother even troubling myself to delete them because the FTP service is already running on your account deleting them will make your site no faster, nor will it make it much more securer - set a good password and just forget about it. Least that way you may want to use the FTP later on.
Most hackers would not even trouble themselves going after the FTP logins since these tend to be more secure and lock out on X amount of attempts also addons domains use the login firstname.lastname@example.org login they would need to work out the domain that is the master which is hard enough to find out. It's much easier to attack the website itself.