signs installed using this plugin can be lifted from the monument by any player using a hammer

Monument Addons already does the right thing to prevent such entities from being picked up. You must have another plugin which is forcibly allowing pickup. I already know your server has that problem broadly because we discussed it in a Car Turrets thread.

https://umod.org/community/car-turrets/52477-the-unauthorized-turret-is-being-removed

Try unloading IQGuardianDrone and XFarmRoom, to see if the issue gets resolved. If not, try unloading even more plugins until you narrow it down. Note that you won't see a hook conflict in this case because the way Monument Addons disables pickup is different than Car Turrets, it doesn't use a hook, but instead sets a property of the object to tell Rust to block pickup. The Rust code honors that property, but any plugins using the CanPickupEntity hook may have been coded to ignore that property (which would mean those plugins are probably coded incorrectly).

This issue can only be resolved by updating the plugins which are forcibly allowing the objects to be picked up.

Thanks for the answer, I will analyze it