You can look up the codes in game by going to Clip board Options, Arena Setup, Objects, Destroy Objects. Obviously you can't "Destroy" the objects online, but it gives you a list of all of them with their corresponding three letter code. Then you just type .dt fxxxYYY where xxx is the field number (add a 0 if it is single digit, ie f01VHH) and YYY is the three letter object code.