Please don't buy that shirt

かわないでください！

How do you ask someone to not do something?

For example, "Please don't buy that shirt. It's scary."

You would take the verb, transform it into short form (ending with -anai), and add でください。

to buy: かう

to buy in short form: かわない (Note that for U verbs we usually we take off the -u and add -anai, but in this case we'd end up with a double vowel, ka-anai. So we must add the わ for smoothness)

please don't buy: かわないでください

Please don't buy that shirt. It's scary. 

シャツをかわないでください。こわいい。