It's a little difficult to comment without seeing the address format your particular web client uses, but this might help...
Some web clients create addresses by converting the user's real IP octet into hex, and using the result as the ident (the webchat at IRCnet.org is an example). If, therefore, the user is on a static IP, then in these cases their ident should remain the same whenever they connect. If the user is on dialup or a dynamic IP, then the hex ident will obviously vary according to the IP block - in theory you could set up some clever ident checks to mark whole blocks, but you won't be able to guarantee that this is a particularl individual and not, for example, their next door neighbour on the same ISP.
Other web clients use the user's real IP as the address and insert a standard ident (Tiscali in Italy for access to IRCnet, for instance). In this case, you're looking at masks to match resolved addresses in the usual way, but again you won't be able to nail an individual on a dynamic IP.
Of course, you could just try convincing them all to use a real IRC client like mIRC instead!