Customize sysctl parameters by user

What @Tomasz says it's true: those are kernel parameters, so they are "unique"!

Anyway, you can achieve some result with that limit the OP, @Osqui doesn't say users are simultaneously logged in...

by sketching out a script executed when users login/logout using the sysctl command

These are kernel parameters and the kernel is shared by all users. As long as your system is supposed to be used by multiple users simultaneously there's no way these parameters could have multiple values at once. And there are normally many various users (personalities) active (owing processes) even if there's only one human user in front of the screen.