A.J.
My 23 month old daughter does this with her drinks sometimes, as well. I find she does it mostly when she's not really thirsty and so she tries to entertain herself and her twin brother. When I find her spitting out her drink she gets the sippy cup taken away. Then somedays she just continues to spit even though it's just saliva it's still messy, so she gets taken out of the highchair and put in the corner. She really doesn't make the mess out of her chair, but for your daughter you may try allowing her only to have drinks in the highchair until you teach her that spitting her drink is not allowed there either. hope that helps :)