Hi Jamen,
Ignoring an alert lasts as long as the alert is triggered. If the alert resorbs, the 'ignore' state is cleared.
You can however set the host to be ignored in the host setup page. This will block all alerts from that host.
AFAIK, there is no other fine grained way to selectively totally ignore a subset of alerts other than excluding the host in the alert checker itself.
Otherwise, if you have the paid version, you can add in a maintenance for that host on specific alerts. This method while working does not feel right to me however.
HTH,
Ahmed.