If there a way to lock a property value of a object from being modified by the player but only by the server?
You can’t in any way stop the client from changing the value. Even if the client wanted to change it, the server won’t see the change from the client. BUT, if it is that important, the best you could do is check with the server each time the value is changed. Although, even then, the hackers (assuming that’s why you’re asking this) can simply delete the script that checks with the server.