Hello and thank you for using JustAnswer. My name is XXXXX XXXXX I will be assisting you today.
The only option for blocking numbers with Magic Jack is with the Do Not Call registry - this is for telemarketers, so I don't think it's exactly what you are after - http://www.donotcall.gov/
You can absolutely change your number - it's very simple. If you log into your Magic Jack account online, there will be an option there to change it at the bottom of the page: