Olivier,
Humbrol spray cans contain an enamel-like paint.
If it is less than a few days since you painted the model, you should be able to remove the paint by soaking it in caustic soda (drain cleaner) for a day or so.
Do be careful with caustic soda: it is harmful to skin and eyes, so rinse the model in water before you try to scrub off any remaining paint.
Caustic soda will also remove most types of putty/filler and prolonged soaking will make polystyrene brittle.
HTH,
-- Eric.