If your Chromebook continually tries to connect to network different than the one you want to use, you need to "forget" network to stop issue.
1) To do this on a chromebook, click on your profile picture in the bottom right of the screen and click on the wifi network (it will say "connected to...and the network).
2) Next, click on the Cog icon
3) A new window will pop up; click on the wifi network again, and then click "known networks".
4) Find the network you want to forget, and click the 3 dots to the right of the network name. Select "forget". Do this with any network you do not want your Chromebook to connect to.