Habbo Hotel is a very safe place to hang out, and the Hobbas (hotel moderators) help to keep it that way.
However, if you're being hassled by someone and need immediate help, you can send an emergency call out to all the Hobbas who are signed in to the hotel. The 'Call for help' function can be found in the help menu - click on the blue question mark (1.) to get to it.
Briefly explain why you need help in this box (2.) - the Hobbas will get a chat log of the last few minutes with your call, so you don't need to give too much detail.
As soon as they receive your call, and if they feel it is a true emergency, one or more Habbos will come to talk to you. If they think it appropriate, they may take action against the offending Habbo/s.
Beware! Only use the 'Call for help' in emergencies - Hobbas are here to enjoy themselves too and don't want to spend all their time dealing with trivial matters. If you've got a general question, please go to the Guest Services Centre - also found in the help menu.