I've finally found how it works.
I've tried against everything and seems doesnt work, but today I've decided to give it more tries...
It seems work against the bane chosen but not against a mongbat, then here is why:
it works only against murders (red name creatures).
Is it intended or is a bug?
Years ago, Dispel Evil "turned" monster spawn (kinda like the old
Dungeons and Dragons Paladin ability "Turn Undead").
After we all got used to that, they suddenly changed it to how it was intended to be: A karma-based, limited range Mass Dispel.
A little after that, unbeknownst to me and as pointed out to me by other posters here (I forget who) they started to let a little of the original, but not intended, use slip in.
Some here suggested, and I think they are right, that Dispel Evil successfully turns any kind of invasion spawn considerably better than normal spawn.
I don't think those Virtuebane guards are red (are they?), but I can turn them.
The bottom line is...This is one of those things in UO that we simply have no idea how it's supposed to work, and an unclear idea of how it works in actual fact.
And trying to streamline it may well simply break something else.
-Galen's player