Hi @AndrewNoble,
This can be pretty frustrating, and honestly, I would just recommend blocking the entire repl.co domain. This would prevent access to proxies (what you are calling non-secure browsers), but it would also block non-proxy sites, and if your students are using it to actually code, they wouldn’t be able to access their pages anymore.
If that’s an issue, firewalledreplit.com is a good alternative, allowing students to continue making their projects, and it would not be possible to create a proxy as it disallows projects from connecting to the internet.
TLDR, blocking the repl.co domain should disallow students from accessing proxies.
If you want some more information on how proxies work, @python660’s guide is extremely helpful. The guide also lists some other commonly used proxies if you want to consider blocking them too.