Excel is an effective selective herbicide that can be used to kill Riccia.
I've not found Excel that useful for Hair grass either.

Generally, for msaller tanks, the Excel idea is useful.
It works well with say, HC.

All liverworts and things without stomata are killed pretty quick, it's not just Riccia, but the Pellia and a red liverwort and another type I've kept all died rapidly.

Regards,
Tom Barr