I am trying to make it when a user is tagged (eg. MyNameJeff#0001), the bot will automatically respond with you're not allowed to mention them and delete their message.
I tried searching for an event that could handle this but haven't found anything useful so far.
You can check if the collection message.mentions.users includes the guild owner's ID with the has() method. A simple if (message.mentions.users.has(message.guild.ownerId)) will work for you:
client.on('messageCreate', async (message) => {
if (message.author.bot) return;
if (message.mentions.users.has(message.guild.ownerId)) {
message.channel.send('You are not allowed to mention the owner!');
message.delete();
}
});