If you are talking about form submissions, then about the only thing you can do is to do the update with your submitted data, [man]unset[/man] the variables in the superglobal array, and then use something like [man]header[/man] to redirect the user to the "success" page.
Depending on how many variables we're talking about here, it usually moves so fast as to appear seamless to the user.
This kind of "trick" can be pretty handy in minimizing form resubmissions.
If you are talking about general code, then you can employ a similar technique pretty easily. (Although there's no way to really prevent them from just hitting their back button.)
