Mirc's internal address list (ial) is not storing realnames, nor gives the on join event that information.

You'd have to whois everyone joining, check the rawlines of your whois reply, and compare the right one it to a list of blacklisted realnames.

e.g. (on unreal, the raw line 311 (whois reply) contains the realname as 6th space-delimited token onwards)
Code:
raw 311:*: {
  if ($istok(Lord.Stupid Dude.Troll.<more realnames here...>,$6-,46)) { mode  <#thechan> +b <banmaskhere> }
}

Note, this line of whois reply does not deliver the channelname in itself.
On unreal ircd, you could set bans directly on realnames, check http://www.vulnscan.org/UnrealIRCd/unreal32docs.html#feature_bantypes