[plt-scheme] How best to create a "sandbox" environment?

Hi folks,

Erich's post about distributed messaging got me thinking about a fun
Smalltalk example that is often used to demonstrated the 'coolness' of
Smalltalk where to clients send code to be executed on each other's
machine (TCP/IP communication, send code, evaluate it on the other
person's machine acting as the server).

It might be fun to set up such an example, but I would like to create
a "sandbox" environment in which this app would run. So for example,
there would be no IO allowed.

What would be an approach to preparing a sandbox that disallowed IO?

I'm pretty sure this functionality is built in but it just escapes me right now.

Best wishes,

Grant Rettke