Categories are just text based options, which can be modified in the language file of the WHMCS module. As long as the category name is less than 32 characters long, any text is acceptable. You may modify the language file of the module to add anything to the list.
Currently in the language file, "spam" must be the third one toward the end of the list. I will change it to "spammer", which actually is a better word to define the issue.