There is a webservice which enables you to delete a message or even a topic
Note that to delete a topic, you will need to first delete its first message
Here are the parameters
user : Pseudo of a forum administrator
pwd : Password of a forum administrator
idforum : ID of the forum where the message is going to be edited
idcat : ID of the category where the message is gong to be edited
idtopic : ID of the topic that contains the message to be edited
idpost : ID of the message to be deleted
Here is a script example based on the message/topic deletion webservice, done in PHP
If everything goes right, the message or/and the topic will return the 2 following parameters
idpostdelete : Returns the ID of the deleted message
status : Returns the status of the operation