Based on my understanding of what rainwolf has said, as long as you are a subscriber nothing would prevent you from indefinitely ignoring. For non subscribers, unresolved invitations bottleneck their game limits.
For non-subscribers the limit is 2, and this limit includes active sets, sent invitations, and received invitations. When that limit is reached, they can no longer receive/send invitations until that number drops below the limit by finishing a set/declining/canceling an invitation.
For subscribers the limit for these is 5, once that limit has been reached, they can no longer be sent direct invitations, but, as opposed to non-subscribers, they can still send out invitations to players eligible to receive them, still send out open invitations, and still accept open invitations. For subscribers, this limit of 5 is mostly to protect them from a flood of invitations.
"For subscribers the limit for these is 5, once that limit has been reached, they can no longer be sent direct invitations,"
I'm not sure if this is functioning properly.
I've had more than 5 KoTH matches going pretty much the entire time I've been playing since I returned to this site a couple months ago, but I've still received KoTH invites.
