He can shift into the Spectral Realm anywhere at any time. He needs to find a shift point in the Spectral Realm in order to shift back into the Material Realm. Plus, time doesn't exist in the Spectral Realm, so trapping Raziel wouldn't work either. Raziel could spend literally decades in the Spectral Realm and once he found a way back into Material, the fight would resume as per normal. Kuja would percieve no amount of time between Raziel's shift into and from the Spectral Realm.

Because of the fact that the shift points are only located in Spectral, Kuja would have a difficult time locating them, considering he can't see them. On top of that, Raziel has an alternate way of shifting from Spectral to Material, but I won't even bring it up unless it becomes absolutelu necessary.