Depends on the type of paint used, and the length of time it's been there.
The first thing I'd try is plenty of hot, soapy water along with one of those green pan scourers, which will be less abrasive than chemicals (not sure on the FZS, but the later FZ definitely uses plastic lenses).
What might also help with some of the more stubborn bits, is a thin, flexible steel rule (one which has plenty of "give", and will bend, rather than dig into the lens).
Bear in mind that if it was "ordinary" paint, some of it may crack and flake in time anyhow (plastic components need plasticised paint, to flex with the plastic when the bike moves), which standard paints can't do.