What do you call a person who "shuts down" ideas?

You know, the kind of person who you tell stuff like "I really want to go to this thing" and they point out everything wrong with it.

Such a person might be a naysayer, "a person who habitually expresses negative or pessimistic views" (TFD)

A wet blanket:

a person who spoils other people's fun by failing to join in with or by disapproving of their activities.

A party pooper?

a person who hasn't the interest or vitality to participate actively in a social party and whose mood, attitude, or personality lessens others' enjoyment; killjoy.

Or for that matter a killjoy?

: a person who spoils other people's fun or enjoyment

A naysayer:

a person who says something will not work or is not possible : a person who denies, refuses, or opposes something

Examples:

There are always naysayers who say it can't be done.
Those naysayers who attack the integrity of the entire judicial system