How do I bind my 'decline vote' key to my ENTER key?

Most servers use a plugin to handle voting, but then others use the built-in TF2 voting to do it.

For the built-in voting you could do something like this in the console, which you enable by going into advanced settings, keys, and developer console (not at my pc right now, can't tell you exactly):

bind "ENTER" "vote option2"

For future reference, there is always this for any questions you might have about keys to bind to, then above it you can find the Binding section. Voting is what I consulted to look up common vote commands. If you want to bind multiple commands to one key you just use a semicolon like so:

bind "ENTER" "vote option2;Voicemenu 1 4"

This will make you pick the second vote option (usually no) and call for a dispenser at the same time when you hit enter. You should customize the vote portion of this keybind to whatever servers you usually play on use for no, if it is not TF2's built-in voting system. Again, some servers use a plugin to handle voting (the ones I play on exclusively do this) and that would require differently-bound keys.

Edit:

It looks like

bind "ENTER" "menuselect 10"

Is what you want to do for any server that uses SourceMod for its voting, 10 is typically the exit key.