I have a young plum tree in fruit (in a large container) and new shoots are very long.Can I trim the ends to stop any further growth?
Sure, it's called pruning and necessary if you want to keep it in the container. Reduce by 1/3rd or 1/2 depending on how long it is.
Yes, I tip those branches starting in early spring and continuing as long as needed. It won't stop growth but slows it down. Usually the 3rd or 4th time I do more than tip. If several branches have developed from the earlier tipping, I may take off all or most of those branches. Then growth may start all over but the tree hasn't gotten taller.