There's a plugin for this. I've not used it, but it looks like just what you need:

http://forums.b2evolution.net/viewtopic.php?t=5258

Ive install the GeSHI Pluging but now i have a problem with the emoticons replacing part of the code inside the <pre...></pre> tags. Any ideas how to fix it without messing up your code?

It's not my code. I've never even used the plugin. If you need help with it, I would ask in the thread about the plugin. Can you just disable the smilies renderer when you're posting code? There's a checkbox in the lower right.