So I've seen lots of reports with no proofs/bad proofs that dosen't proof much. As a player that reported many times I've learned some stuff that can help me, and I'm sure will help you, to get good proofs. Here's the tips that helped me to get good proofs: 1) Don't wait a few minutes to take the screen shots, if the chat is full of spam, even if you will scroll up you might not see the thing that the person said and you will have no proof. 2) Always get the program you're taking screen shots with open, so you could take screen shots everytime. 3) If the person you want to report did something like advertising, and he didn't spam the IP like, he only said it once, and the chat is full of spam and you might not see again the message he sent (like I said in option 1) Don't say stuff like "reporting" because in some cases the person might get scared and won't keep the advertising, and you will have nothing to take screen shot of.
You can press F2 in Minecraft to take screenshots. Go to your .minecraft folder to access them. The pictures should be in the folder named screenshots. To add on this, you can just press whatever your talk button is and scroll through the chat to get a good picture of the ip address. Note that after a certain amount of messages after the advertisement took place, the messages will disappear from the chat
what i ment is, that even if you scroll up after you press the chat button you might not find it because theres sometimes so much spam (like in creative for example)