How do I spray paint Yoga balls without them getting sticky?

Use Krylon Fusion spray paint for plastic or any other brand. Several light coats will give you a smooth finish. Let each coat fully dry.

What is your purpose in doing this? What will you be doing with the balls, and where are they going, once they are painted?

You need to find out the what material the balls are made of and look for a spray paint that is safe for that. Years ago I tried to paint a pair of rubber rain boots with spray paint and the paint never cured. I think this is what happened to you because the paint is not compatable with the ball.

the balls are flexible 'rubber/vinyl' etc. even if you get them to cure properly, any flexation will crack the paint .