/Ignore is for maintaining the list of filters. Hostmasks are usually entered automatically by the flood protection, but you can register filters yourself for specific idiots.
/Ignore on its own will list all the filtered masks.
/Ignore \<hostmask\> will tell you what will be ignored from a given hostmask.
/Ignore \<hostmask\> +|-\<types\> will add/remove specific events to/from the list.
Instead of \<hostmask\>, you can use the list index (given as a number).
*
/ignore \*sex.\* +\*
*
would ignore all message from anyone with 'sex.' in any part of their hostmask.