should I change my seatpost? I have a seatpost with spring suspension. Something like this one: 
A friend of mine hit it with his leg while he was dismounting and broke it into two pieces. After putting it back, I noticed that the plastic piece that holds it together has a broken thread. The bike is about 1 month old, so it is still under warranty. 

Should I leave it as it is and hope for the best, or should I go back to store and ask for a new one, alleging bad manufacture? 

Also a plastic ring it is clearly to weak for that join should I just get a simple seatpost ?
If you prefer the cushion of the seat post, you could request a new one, especially if it keeps coming apart. It should be covered under the warranty. If you don't care about the spring support, a simple seat post will be more sturdy.