Roku premiere remote will not turn LG42Lk450 tv off

Km94-

Simplex is what LG calls its implementation of the CEC (Consumer Electronic Controls) standard for receiving commands over an HDMI connection.  Many TVs, even those with some CEC support, do not respond to the full set of CEC commands.

To see if your TV supports the CEC commands used by Roku and thus set the remote up to use CEC commands, go to:
Settings > Remotes & devices > Remotes > √ {Your current remote} > Set up remote for TV control > CEC setup
... and follow the prompts. If the required CEC commands are not supported by your TV, it will let you know it failed.

If it cannot control by CEC, your remote can still control via infrared (IR) but it must be set to the correct set of IR control commands as is used by your set. 

Roku menu path:
"Settings -> Remotes & devices > Remotes"
Select your Remote (in use) then select "Set up remote for TV control" .. and follow the prompts.

Your LG likely has several sets of possible IR command sets.  It may be that the first one that turns the sound off when going through the above routine, only turns the set on but not off.  Try repeating the remote up for TV procedure again, this time lying the first time the sound goes off so that it goes on to test the next set.  See if the next one that turns the sound off results in both power on and off controls.  If not, repeat the process.