You need to remove the flowering stem as low down as you possibly can. Some varieties of rhubarb are more inclined to bolt than others it can also be a sign that the crown is stressed, if you are in the drought area of England have you kept it watered as it is a young plant?

I agree with Moon growe

I prefer to cover it after it has died back with a thick layer of farm yard manure, this retains the winter rains and the moisture does not evaporate as quickly.

Then when I pick it I cover it with custard! ;)