The spammers that register are automated bots that perform various tasks without much, or any human activity (register, confirm account, post message, send PM). Some are as innocent as posting links to "cheap air jordan sneakers" and others are more malicious (attempting to infect or send people to infectious sites).
We don't allow HTML in PMs or messages, so the ability to a spammer to cause harm is limited to when our users click on the links that they post. Obviously if the spammers are clever and disguise links as relevant or useful, it becomes more likely that some users will click on them.
When we become aware of spammers, we delete all PMs they sent, all messages they posted and delete the accounts they used. It's highly unlikely that any spammer uses a traceable IP address, since their activities are illegal in most countries.