Don't answer back... Immediately... Wait a few hours or for the next day and then reply you were busy. 2 or 3 times should do before the other person gets the picture
How to get someone (who crushes on me) to stop texting so often?
Tell them to stop.
Then file harassment charges against them, if they continue.
Use Walt's if you want to do it quickly and possibly offend him/her (not saying that that's necessarily a bad thing)
Or say you need to go and do something, or just put their texts on Do Not Disturb or block them altogether. If they ask you about it, tell them you never got their texts, and that you'll check your phone for them later