WP-Cron is hacked sends out thousands of spam

help.
I have looked everywhere, but I am a WP user, not a techie.
1.) I have now 72 WP installations.
2.) my server has been hacked through wp-cron and I was shut down because my server was mailing out a lot of spam like viagra and porn
3.) I was reading that I can disable wp-cron in the wp-config file using this code

define(‘DISABLE_WP_CRON’, true);

next I am to activate a true cron job but I cannot find out where and how to exactly do that, I need help please.
The company where I lease my server from also tells me to install a special code into wp-admin> .htaccess to block access from any IP other than my own.
Is there a WP plugin that can take care of the wp-cron problem and also a WP plugin which can write into the .htaccess file?

Please, if someone out there would help me I really be grateful.
thank you
Michael