As a GM it is my job to see this and to shut it down or fix it as I go, this had lead to some pissed off gamers who have been mad at me. But to be honest if they are using a exploit and know it then there wrong and are cheating and don't want them any way. With that said I would go with letting them pick the spells off the bard list they want as to other spells from other sources, roll at random to see what spell they could learn don't just let them cherry pick that is too much of an advantage and we all know this. Make them do research or have to find the knowledge to do a spell if they know of it or have to find some one to learn it from or to gleam the knowledge from. Even then I think some kind of roll to determine if they learn it should apply, just thinking out loud here. I also think some spells should not be obtainable by a bard and be class only to as to protect the class uniqueness. I will have to look at the bard more closely to make sure there is or is not an exploit as I am very good at spotting them and my players hate me for it.