Is there a way to exclude specific hosts from a Nagios hostgroup?

Add the specific host with a ! prefix in the service definition. The service check below will apply to all hosts in agroup apart from ahost.

define service {
    hostgroup_name   agroup
    host_name       !ahost   
    service_description Shared NFS mount
    check_command check_nrpe!check_shared_mount
    use generic-service

}

Haven't yet tried it with hosts, but with hostgroups, prefixing with a ! works. I use this for running a different load check on busy servers:

define host {
    use         physical-host
    host_name   busy-host.example.com
    alias       busy-host.example.com
    address     10.43.16.1
    hostgroups  linux,centos,ldap,http,busy
}

define host {
    use           physical-host
    host_name     normal-host.example.com
    alias         narmal-host.example.com
    address       10.43.1.1
    hostgroups    linux,centos,dns,proxy,ldap,hp,http,puppetmaster
}

define service {
    use                   generic-service
    hostgroup_name        linux,!busy
    service_description   Load
    check_command         check_snmp_load
}

define service {
    use                   generic-service
    hostgroup_name        busy
    service_description   Load
    check_command         check_snmp_load_busy
}