With my oldest we trimmed the nipple at 18 months and he threw it away himself, and only asked for it a few times. With my youngest he kept chewing on them and I kept telling him you are gonna break it, and he finally bit a hole in one (like his tooth went through) but it was enough for it to lose its suction, for air to get in, and he carried it around for a couple hours going 'broken' and finally he threw it away, and when he'd ask for it I'd say 'waht happened to nigh nigh' and he'd go 'broken, trash' and that was that. ITs been 3 days now and he's fine. Doesn't ask for it at all. He's 20 months.
Id' cut the tip and just be all 'uh oh! ITs broken, time to throw it away' and let him toss it on his own terms. Let him hold it and keep if he wants to, he'll lose interest if its not working right, with a hole in it, cut a small piece off the tip so it won't have suction. He's big enough wehre he'll understand its broken and its in the trash.