disable upstream file directory browsing

We have several clients that access a specific folder that is based on
their login. The problem is that all of these folders are inside of
one large 'customer' folder. We want to make it so that once they
select the link to take them to their specific folder, they cannot go
back upstream to the main 'customer' folder. I was told that
mod_rewrite can accomplish this, but I cannot find any examples that
show what I'm looking for. Are there other options out there for me?