If you can't say anything nice don't say anything at all

i have a question for you: is there any way of politely be agressive to anyone?. would be the next improvement in military operations to be albe to war against someone politely? anyway, is there any need to be polite all the time?

You can say, "Your ideas are interesting but I just don't agree".

First of all, there is no nice way to insult people, but I guess you can cushion it a little if you really feel the need to. Like using negative modifier on antonyms to sound a little less insulting. Like "Not smart" instead of "stupid/dumb/retarded/etc" Also directing the comment to the idea instead of the person may be better approach. "Not exactly the brightest idea you mentioned"

In my point of view there's no right way of telling somebody they are stupid. If you don't agree with their behaviour/opinions tell them politely.