This page explain how you part channels in IRC
There are different ways of parting channels depending on what client you use to access IRC.
A common way is through text commands though. Such a command look like
/part #channelname
and can be written either in a eventual status window or another channel. If it's different in your particular client, you should receive a message about this from your client, helping you to do it right.
To part multiple channels at once, you can enter the channels in a comma-separated list
/part #channel1,#channel2,#channel3