Acetone does not "melt" plastic. Melting means a physical change from solid to liquid caused by an increase in thermal energy.
What acetone does to (certain) plastics (e.g. PMMA) (there are hundreds of synthetic polymers which can theoretically be called "plastic" and not all of them are affected by acetone) is to simply dissolve it.
However, if you wash the acetone off, then no further solvation of the plastic surface will occur, although at normal room temperatures the acetone will rapidly evaporate from the surface of the polymer anyway, and even the solvated surface will rapidly dry.
Acetone is miscible with water, and you do not really need anything else to wash it away from a surface.
You ask what can be used to "neutralise" acetone so that it does not continue its "corrosive process". Acetone is not corrosive to the polymers it dissolves, any more so than water is corrosive to salt or sugar - it just happens to be a good solvent for them.
"Neutralisation" would refer to acidity or alkalinity - if you are asking for something to be added to the acetone to prevent it from being a good solvent for specific polymers, then the answer is simple: water. Wash the surface of the polymer with water, and any remaining acetone will be flushed away.
Professional chemist since 1987. >15 years experience with structure, properties and formulation of polymers and polymer/material blending.