Axel,
In the .htaccess file in your forum root (or you can put it the .htaccess file that's in your public_html folder, to make it site-wide), add the following lines *after* the "DirectoryIndex" line :
RewriteCond %{HTTP_REFERER} !^http://mydomain.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://mydomain.com$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.mydomain.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.mydomain.com$ [NC]
RewriteRule .*\.(jpg|jpeg|gif|png|bmp|avi|mov|wma|wmf|wmv|doc|pps|xls)$ /leecher.html [R,NC]
Substitute "mydomain.com" with your own domain name, but leave the remainder of each line as shown.
I created a page titled "leecher.html" and placed in public_html, so that's why the last line references it. You can use any page name you wish, as long as it's in the public_html directory for that Domain of yours, then change "leecher.html" to the name you've used.
It's one easy option, anyway, and prevents hotlinking of all the filetypes listed in the last line. You can add filetypes as you see fit, but follow the example as shown.