You can do that using Joomla; there is no need to disable the plugins previously. Just go to the extensions manager, look for 'security' and then select all the Once listed or only the component. This will completely remove the component, plugins and modules.
Now that I have followed your advice, security check pro is well uninstalled but I still have a blank page in front and I cannot go back. "System - Securitycheck Pro Cron" was a problem for me ....
Cron plugin only launches file integrity and/or file permissions task... If do you have a blank page even if Securitycheck Pro is uninstalled then you should look for more info in the error.log file of your Apache.
I reinstalled a backup of the site. Again, when I deactivate "System - Securitycheck Pro Cron", I have a blank page in front. If I disable all securitycheckpro elements except cron, everything works. If I uninstall securitycheckpro, system securitycheck Pro Cron is uninstalled and a blank page is in front. I doubt that it comes from the server, I tested on other sites hosted on the same server and disabling securitycheck cron is no problem. How can I do this and what is happening on this particular site? Thanking you
That's really odd; maybe there is a reference to cron file (with a 'require' or into database) and that's causing the blank page when you uninstall it.
Try setting the 'Error reporting' to 'development' into global Configuration of Joomla. If this doesn't give you a clue of what's happening then send me administrative credentials to access to the cPanel (or phpmyadmin and ftp); use the email from which you receive forum notifications.