The Scratch that I am familiar with does not use a Web Browser at all, so it should be no trouble. I will admit that I have not looked at Scratch on Linux at all (not my Operating System of Choise), so something may be different there.
Just remove Midori, Chromium, and Netsurf from the default Raspbian (noting that all three are normally installed by default, and it is easy enough to run them from the command prompt).
You could also just not connect the RPi's for that purpose to the internet at all (get models without WiFi, and do not hook an ethernet cable to them).
RPi = The best ARM based RISC OS computer around
More than 95% of posts made from RISC OS on RPi 1B/1B+ computers. Most of the rest from RISC OS on RPi 2B/3B/3B+ computers