How can I keep my mums in a round ball like when I bought them

Put some string around them if you want, but their charm is in spreading out.

You must keep pruning and I mean pruning a lot. Twice a week or better to keep it from forming buds. You do this from about mothers day till the fourth of July and then let it go. You will have a great show of flowers in the fall. If you need string you failed and let it go.

I mean, pinch the new growth, as you would any plant to make it more
Dense, not leggy.