Hi,
This means that the computer cannot find a valid system. There may be several reasons for this.
You could try to press and hold the Shift key at startup to see if there is any change. You could also try to press and hold the four keys Command(Apple), Option(Alt), P and R immediately at startup (hold until a second startup sound can be heard, if any).
You would need an appropriate bootable tools CD (system disc) in order to (securely) erase the hard disk.
Do you have another Mac with a FireWire port? If so, FireWire target disk mode could be used to gain access to the internal hard drive of the iBook.
If nothing else helps, you may have to open the iBook and remove the internal hard drive (try a web search for details). The removed hard drive can be connected to another Mac (or possibly a PC with software for Mac disks) in an attempt to read the contents (and/or erase the hard disk). For this you would need a universal USB bare drive adapter such as the example at https://eshop.macsales.com/item/NewerTech/U3NV2SPATA/). Alternatively, keep (or destroy) the hard drive and recycle the computer.