how to disable users from using IE7 and below
 

I'm looking for a method to disable users from using ie7 and lower versions so that my forum get low level spammers attack since old ie versions known to be security flaws...

is there a plugin or script in order to achieve that??

You can use the 'User Agent String' to detect IE7 and lower. However, anyone who is trying to do you harm will just spoof the 'User Agent String' (forcing the browser to look like another browser). You can also perform feature detection in the browser using JavaScript. JavaScript of course can be shutoff unless you devise a method for it to be mandatory -- one approach is the Google gmail method.

Basically, you use all three methods. I am not sure there is a bulletproof way, but you can get close. The terms I used in the above paragraph can easily be Google'd to receive a wealth of information.

PS: You should be ridding yourself of IE8 too, if your going down this route.

Bad browsers affect users, not your site.
Your thinking on this is wrong.

Blocking spammers from joining has nothing to do with browser versions.