Vandals appear in RCT1 and RCT2; they also appear in RCT3, although they are less frequent and present fewer problems.
Vandals are guests that are extremely unhappy with the park's state (generally from excessive vomit and litter), and destroy path items such as benches, lamps and trash bins. Vandals are characterized by an angry red face. In the guest list, the icon next to their name will look like a red face with a frown.
A good way to see if you have a vandal in your park is to turn all rides to see-through and making guests/scenery invisible. Any broken path scenery (i.e. Benches, litter bins, street lamps, etc.) has been vandalized. Guests that pass by these objects will often comment "The vandalism here is really bad", lowering the park rating. They can be replaced easily by clicking on the item with a new item such as a lamp or bench (place it as you normally would).
Note that you cannot tell a vandal apart from others by simply looking at the walking guests. They look identical. Also, if they are sick, they will still have the red face, even if they become less nauseated or happier.
Security Guards counter vandal activity. [clarification needed.]
To calm down the guest, carry the guest into a 3x3 pathway away from others and place benches and lamps just in case. They will calm down after a few seconds or minutes.
Another (and much easier) way to remove a guest's vandal status is to change their name to "Melanie Warn", increasing happiness and energy to max. This can work with all guests that a player wishes to make happier. You can use the name more than once by changing the capitalization of letters (i.e, MElanIe WARn and MELANIE warn will count as two separate names, but they both have the same effect).
A third (but much more brutal) method to deal with vandals is to simply pick them up with the pincers and drop them into a body of water.