Nope. At least not as far as I know. I would assume it is because *spoilers* you contact them during the dark brotherhood questline and if you destroyed them and then you did the dark brotherhood you wouldn't be able to complete it unless they set up someone else somewhere... but I was really hoping you could myself since there are those 2 people talking about wanting to make riften a better place when you walk in for the first time.
You could destroy the dark brotherhood, so why I don't know why Bethesda wouldn't let you destroy the thieves guild. <http://www.uesp.net/wiki/Skyrim:Destroy_the_Dark_Brotherhood!> <http://elderscrolls.wikia.com/wiki/Destroy_the_Dark_Brotherhood!>
*End Minor Spoilers*
The two people are Mjoll and Aerin right? I was thinking the same thing as you. I thought Mjoll the Lioness would be the person that helps finish the thieves off. More specifically, I thought she would be the rival of Maven Black-Briar. I wanted to fight Maven through a quest, not just randomly murder her. I decided to join the thieves guild to see if there was a way to overthrow Maven. After doing a number of the main thieves mission, I don't think she will be overthrown.
Been writing on Gamefaq about this problem since the 2nd day of the game launch. I really hate Maven, I mean, I'm okay with doing the Thieves guild quest, but really want to get rid of the Black Blair family. Much like you all, I thought Mjoll and Aerin were the key to fix Riften, I even got the sword back for Mjoll, but there was NOTHING!
To make things worse, you can't kill the Black Blair family. They are marked essential. Someone really need to fix Bethesda's error and make some remedy to Riften's situation.