It's something that I have never seen come up during turf play, but it happened to us thanks to messy subbing. Neither team knew exactly what the rules were so we just removed the extra player and continued on.
I've found something that is a rule in another league http://www.mcgillathletics.ca/custompages/CREC/Intramurals/ultimate5on5I... as the vul rules are for 7on7.
If a substitution results in too many players, of either gender, on the field, the result is an automatic turnover and the defensive team takes possession of the disc at the area of the stoppage of play due to the violation.
Is this the rule we should follow?
What should happen if the offensive team calls it against the defensive team (I have the disc and call it against the other team with +1 player)? Just a stoppage in play for the player to get off?