Sexual Harrassment?

If you go to settings > and then the privacy tab you can enter
names you want to ignore into there. You can also change
how you want to receive messages, shouts etc.
http://www.last.fm/settings/inboxoptions

Also you can use this:
What can I do if I want to report an offensive user or inappropriate material?

How do I prevent unwanted messages from strangers?

You can prevent strangers from contacting you on Last.fm by adjusting your privacy settings. ( http://www.last.fm/settings/inboxoptions )

Under the heading “Receiving messages”, change “Accept messages from: Everyone” to “Friends only”.

How do I block a specific user?

You can prevent specific users from contacting you on the site by adding them to your ignore list.

In your privacy settings (http://www.last.fm/settings/inboxoptions/) under the heading “Ignore list”, enter the username of the account you want to block, and click the “Add to list” button.

Users on your ignore list will not be able to:

•Send you private messages
•Post on your shoutbox or journal
•Share tracks, albums, or artists with you.

Their comments will also be hidden on all shoutboxes and the forums.

What should I do if another user sends me threatening or inappropriate messages?

Last.fm takes these matters very seriously. If you feel you are the victim of threatening or abusive messages (including stalking and bullying), please do not hesitate to contact us here: http://www.last.fm/help/support and our moderation team outside of office hours. All abuse reports will be treated in confidence and investigated thoroughly by our support team.