To hide such subnets from IPAM Summary page 'Top 10 Subnets by % IP Address Used' specify filter Subnet.CIDR < 30
Similar filter can be used in Advanced Alert Manager to disable alerting on /30 subnets.
You can also, and this option is a bit dirty, put subnet address and broadcast address into available state which will affect calculation and report 50% usage then.
I love complete and effective answers. Thank you romkon.