You can report problematic posts or profiles by clicking their ⋯ button and selecting "Report".

Remember to include links to problematic posts in your report. This saves the moderators a LOT of time, and means the moderation will happen much more quickly and accurately.

If possible, select the option to anonymously tell the account holder's server too. They can shut down the account if necessary. (However, if their server is problematic too, it might be best not to use this option.)