There is any IRC command to ignore a person

you send it to a 'service', a special purpose bot. and you can send that message as soon as you are connected to the server.

• click to "chat.freenode.org"
• click 'console'
• enter:

/nick yournick
/msg NickServ register yourpass yourmail

It depends on the IRC server and the channel. Some IRC servers provide this service and some don't. Some channel bots provide this service and some don't, and some channels have channel bots and some don't. The easiest way to find out if there is a way is to ask in the channel. Usually, there isn't.