Ask them to explain what problem they're trying to solve. If they say "we might need it later" or "just in case", then they aren't adhering to YAGNI. In general I've found that sticking to principles at least makes it easier for people to accept. It also sets expectations as to what you won't accept.
I've also found that agreeing to revisit the idea to help. Acknowledging ideas is important.
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Ask them to explain what problem they're trying to solve. If they say "we might need it later" or "just in case", then they aren't adhering to YAGNI. In general I've found that sticking to principles at least makes it easier for people to accept. It also sets expectations as to what you won't accept.
I've also found that agreeing to revisit the idea to help. Acknowledging ideas is important.