Focus
Focus is an app that can block websites and apps. It can block them forever, or for a set period of time.
This is useful when you need to remove all digital distractions so that you can work on what’s most important.
This can also be useful as a temporary sanction for students who have demonstrated that they struggle to resist the temptations afforded by some tools on their laptops.
As a temporary measure, it is a means to an end. Ultimately we want students to be 'Self Managers' and use self control; or as we explicitly state with our Expectations, that they should demonstrate their RESPECT for others, and how to use the device they use RESPECTFULLY.
Blocking additional sites...
The Focus app already has a fairly comprehensive list of websites to block (blacklist), to add new culprits, all you need is the URL for the site, but it's needs to be the root domain not a subpage,
eg reddit.com/ not https://www.reddit.com/r/aww/comments/j2987i/halloween_dog/
So, to block Google Chat you'd need to add chat.google.com/
You may also need to block chat.google.com/dm (for browser shortcuts)
GMail is already blocked by default (as chat can be accessed from within Gmail) mail.google.com/ your students shoudn't need to use GMail in school, as all class messaged can and should be sent as a Seesaw Announcement.
Warning
In order to stop students from simply overriding your restrictions and—even worse—setting their own password so you can activate the Focus App again, you should activate Hardcore mode, and Password mode.
Lockdown
This way, once you have activated Focus there is nothing they can do to deactivate it, but be careful, as there is nothing you can do either, so be careful with the length of the Focus session, at least for the first few sessions.
For these reasons, lockdown is best for you to do for the student, not with the student...