can u help me with text message spam??

I recently purchased an Iphone. in the past couple weeks i have been getting text messages that are complete spam. They say things like: Zoosk mail: youve got a flirt. And I would block the number, but the messages are from all different numbers. I have recieved 44 text messages from this sender in the couple hours. The text messages are costing me 50 cents each. It adds up. The number is always different but it always starts with 990-**** . Your help would be GREATLY appreciated