I think that a commont common technique for proving such statements is for example the following type:
On
One class shares a closure property, while the other cannot because of a hierarchy theorem. Thus the they cannot be equal.
In this particular case a proof should could proceed along these lines: Since NP is closed under polynomial time reductions, so would SPACE(n), if they were equal. Then deduce that polynomial time reductions would imply that SPACE(n^2) is contained in SPACE(n), which is impossible by the spache space hierarchy theorem.

