Different between self:: and static (Late Static Bindings)

Static references to the current class like self:: or __CLASS__ are resolved using the class in which the function belongs, as in where it was defined.

Method 1:

Static cache Control by .htacces

If not already done, you must enable apache headers (mod_headers.c) module

Check module is enabled or not:
ls /etc/apache2/mods-enabled/

Method 1. (Classical way)

Use a captcha system like Math captcha, Image captcha, reCaptcha of google....

Media Print Emulate on Chrome

(Tested on Chrome Version 56)