Anti Spam Plugin

Features

Block incoming subscription requests
Block lengthy messages
Strip XHTML formatting for group chats/private messages from group chats
You can set up an anti spam question that has to be answered before someone can contact you. This is applies for everyone who is not in your contact list. Additionally, you can enable it for private messages in a group chats. Note that all message content will be dropped before the correct answer is given.

You can install the Plugin via the Gajim Plugin Manager.