[plt-scheme] Preventing built-in functions to be redefined

Hi

I am fairly new in Scheme.  May I know if it is possible to restrict
the redefinition of the built-in function, or if possible, make any
other user-defined functions non-writable?  Pardon me for being a
little paranoid, but the fact that (define define 5) actually making
"define" equals to 5 is just somewhat alarming to me.

Thanks.
nandehutu