Why would you need it? Just set it to 0 in server.cfg and amxx.cfg. Don't give change cvar access to your admins if they'll do it. You can use this, it will set friendlyfire 0 on every round start.
PHP Code:
/* Plugin generated by AMXX-Studio */
#include <amxmodx>
#include <amxmisc>
#define PLUGIN "Anti-FriendlyFire"
#define VERSION "1.0"
#define AUTHOR "WhiteFang"
public plugin_init() {
register_plugin(PLUGIN, VERSION, AUTHOR)
register_logevent("new_round", 2, "1=Round_Start")
}
public new_round()
{
//Setting Friendlyfire 0 on new round
server_cmd("mp_friendlyfire 0");
}