There's no report tab for someone intentionally trolling you in game.
To start - this isn't entirely true. Under Negative Attitude, "Griefing" is listed as a subcategory of it. I advise that, if you report players for griefing, use the Details section of the report page to outline their misbehavior for Riot to take into consideration for future punishments.
That aside - I've experienced this problem more than I should have, considering that as of late I've been focused pretty intensely on Jungling. Whenever someone starts stealing my camps, I politely mention that I don't appreciate them doing it. If they do it regardless...Well, I have options. All of which are fairly petty, and I wouldn't recommend them, but they're not particularly bad, and they're not game-changers, so I can only assume they don't fall into a punishable territory.
In other cases, I've been inclined to make half-hearted jokes to players who steal my camps consistently. Something along the lines of "if you really wanted Jungle that badly, you could've told me in champ select" - it doesn't stop them, that's for damn sure, but it at least dissolves a little tension in the situation. At least, for me. Not sure if it'd work for you.