This simple one-liner works for me-
hook.Add( 'PlayerDeathThink', 'NoRespawn', function(ply) return false end )
From the wiki: “Overwriting this function (PlayerDeathThink) will prevent players from respawning by pressing space or clicking.” (Note that using a hook doesn’t actually overwrite the entire function, it just gets called whenever the PlayerDeathThink function is used, and returning a value for this specific hook (true or false) overrides it, meaning the players cannot respawn. (I haven’t tested if it’s actually possible to respawn in different ways however, like with console commands or something)
Or, if you wanted them kicked, you could do:
hook.Add( 'PlayerDeathThink', 'NoRespawn', function(ply) ply:Kick( 'You died!' ) end )
This one uses the variable ‘ply’ which is returned from the PlayerDeathThink function, so you could use it to kick the player!
Sorry for spoonfeeding, but you seem to have already tried a lot…