I agree that although 'riserva premi' normally translates as 'premium reserve', in this case it refers to the 'riserva premi a carico di riassicuratori' therefore the translation changes to 'unearned premium reserve) (sometimes known as the 'reinsurance reserve').

In Insurance 'storno' is equivalent to 'return premium'.

This is an entry from a leading insurance glossary:

return premium

The amount due to the insured if the actual cost of a policy is less than what the insured has previously paid, e.g., if the limits are reduced, the estimated exposure at inception is greater than the audited exposure, or the policy is canceled.

And an explanation from Wikipedia:

Return Premium

When a policy is canceled before its expiration date a return premium may be owed to the insured. The return premium is generally calculated using a wheel calculator. The return premium is calculated by calculating the unearned premium and then subtracting any unpaid premium and penalty for early cancelation. Short rate (old short rate) and short rate (90% pro rata) are penalty methods of calculating the return premium.

http://en.wikipedia.org/wiki/Earned_premium#Earned_Premium

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Note added at 1 day7 hrs (2011-08-06 22:59:40 GMT)
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And an entry from an Italian insurance glossary:


STORNO
Operazione contabile che consiste nell'annullamento di una partita relativa ad una rata di premio o all'importo che risulta da un'appendice di polizza, per l'impossibilità di conseguire l'incasso o per altri motivi (sostituzione della polizza ecc.).

http://www.assor.it/Glossario/s/storno