robots.txt is a way to stop most robots from visiting your site, most will respect your choice to NOT allow them to visit your site.
Simply create a file named robots.txt and enter:
User-agent: Mozilla/5.0 (Twiceler-0.9 http://www.cuill.com/twiceler/robot.html)
Disallow:/
Keep in mind that most bots have a cache of your robots.txt which they update on a regular basis into their database, i believe Twinceler uses a 7 day cache.
If you wanted to block them by say a .htaccess file (if you're on an apache webserver and have applicable permissions), you could do so as:
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} ^Twiceler
RewriteRule ^.* - [F]