Hello, Mr. Hahn. Depending on the context, "grass" can be either countable or uncountable. In this case, it is uncountable and therefore your first sentence is correct: "Please don't park your car on the field. You might spoil the grass!" An example of the word "grass" in a countable context would be: "Cows will eat almost anything, including many different grasses."

I would say: "Please don't park your car on the field. You might ruin the grass!" or even better: "Please don't park your car on the field - it may ruin the grass!" and no..."grasses" is not a word.