Viber for iPhone was refreshed this morning in the App Store with improved spam handling. I've noticed an increase of spam messages on Viber lately as I've been regularly getting spam messages from seemingly random people telling me I've won a free iPhone or some fishy contest prize.
While Viber offers the ability to block unwanted contacts from messaging you, this update should help sort out the influx of spam on the service.
No other changes are mentioned for Viber 5.5.1, a free download from the App Store.