Vandalism is the act of deliberately and intentionally disrupting or destroying another's work for personal and/or group-based motives.
In-game vandalism is considered a commonality on Roblox, and happens in many different ways at any given time.
Spam through copying bricks, models or any other in-game object is considered vandalism as it lags the game server and is meant intentionally to slow down the server, crash players, or to ruin other's creations or general game play.
Deleting acts in the reverse way, as bricks can be deleted to where the users work has been partially or completely destroyed and removed.
Exploiting, although generally less common then the other two, can also bring destruction to servers, where they are able to edit places and usually spam or delete on said server.
Hacking (or phishing) to get another's user account is a common form of website vandalism (remember, any link to "free robux" or " BC for free", and anything else that asks for your login credentials should be considered an attempt to steal your account).
Spam through posts or messages is considered vandalism to one's inbox, threads or the forum it's taking place in.