I don't want my cron type plugin to run at plugin activation

I don’t want my cron type plugin to run at activation, but no matter what I’ve tried, it still runs when activated. The plugin removes spam from the comments table, and I was hoping to allow a user to configure it before it does its work. The register_activation_hook function doesn’t return anything, so it can’t be checked. is_plugin_active is giving me a function not found error, and when I try to set a private variable to flag as an activation, it doesn’t do anything. It’s as if after the plugin is activated the plugin is reloaded as active. Should I worry about this, or is there a fix?