Quite right. Thus, you will ignore private messages from any users who have this domain zone "ch" in their host address.
Or, for example, if you want to ignore messages from all US users, use this address mask: /ignore -p *@*.us
The "-p" switch next to the command tells mIRC to only ignore private messages, but you will still receive other types of messages, so if that's not enough, a few other switches can be added.
Note: You should understand that by using such a wide range of ignoring addresses, you run the risk of not receiving an important message from your friends or new acquaintances who have the same domain zone in the host. In this case, you can add some addresses to the exclusion list using the "-x" switch.