Hi Daniele!
What you can try is heating a utility knife, or better some type of exacto knife, as these blades are usually pretty thin. I don't know what kind of plasticine you are using, but usually they melt when exposed to heat, so a hot blade to cut up your sculpture could be the solution once it has cooled down.
This is what I do when I need to cut (hard) Castilene or Cx5 once both materials have cooled down, because you can't use a wire cutter with those two either.