There are even more ways to kill a new-born idea. Here are just a few. You can probably fill in the rest.
Say nothing. Pretend that it was not said. Let it die in silence. Most people can take this hint that they have said something that is not wanted.
Say how bad an idea it is. Show yourself to be an expert in such things and that the person offering the idea is not an expert and hence unable to offer any idea of any use.
Damn it with faint praise. Say how very interesting it is. Or how it might just work (with the wind in right direction). And by implication how useless it is.
Laugh at it
Smirk, giggle, laugh, chortle. Say how funny the idea is. This will be very effective at preventing anyone else taking it seriously.
Ask searching questions about it. Of course, as a simple idea, the person offering it will not have thought it through. If you probe far and fast enough, you can tie them up in knots. Then just leave it hanging, like the lawyer who has just crucified a witness.
Say how the idea has been tried before and found to be completely ineffectual. In doing so, you show how the person in question is only good at coming up with ideas that are of poor quality and are old hat.
Compete with it
Come up with a better idea that shows the idea to be not that good. As the new idea is yours, you can either now get the credit or drop it quietly as it has done its job.
Take up the idea enthusiastically and adjust it so that it is no longer recognizable. As
Shoot the messenger
Instead of attacking the idea, have a go at the person bringing it up. Tell them that they are no good at creating ideas. That they should not be offering such thoughts here.
This is particularly effective at putting off other people from offering ideas.
Give it to a committee
This is the death kiss for almost any idea. Ensure the committee is made up of people who will argue or delay and end up with nothing in particular being created.