My character picked up an "unusual gem", which triggered the quest "No Stone Unturned". This quest cannot be completed without joining the Thieves Guild, which my character would never do. So, I've removed the quest from my journal by using the command "setstage tgcrown 20". However, doing this gives me the quest reward, which is the thief ability "Prowler's Profit".

What do I type in the console to remove "Prowler's Profit" from my character? Alternatively, is there a way to remove "No Stone Unturned" which doesn't give me the quest reward in the first place?

Thanks for your time.

Use wiki or the UESP(?).
I doubt anyone knows that specific console command off the top if their head.

I'm not entirely sure you need to join the Thieves Guild in order to do the quest. I think you only need to join the DB and College of Winterhold factions to gain access to gems.

Edit: Scratch that, just read the wiki on it. I guess not being able to remove the quest from your journal would be rather annoying.