A kickstarter project cannot go under funding goal in the last 24 hours if it has already reached its goal. You can back out if you contribution doesn't bring the project below goal. Part of the anti-trolling rules.
Backers may increase, decrease, or cancel their pledge at any time during the fundraising campaign, except that they may not cancel or reduce their pledge if the campaign is in its final 24 hours and the cancellation or reduction would drop the campaign below its goal.
What about reducing the contribution?