For better or worse, they will not add an “undo” or “ignore” button to the website. The creator of WaniKani has said time and time again (including just two days ago to me via email) that this is against their core philosophy.
The mistake detection is more lenient as the correct answer gets longer, but for answers that are only three characters, no mistake is allowed. If you want, you can install the override script (which let’s you ignore wrong answers and try again later) or double check (which allows you to mark you answers as right, wrong, or try again right away).
As Ninkastmin said though, you have to use them wisely. If you use them too freely (like “oh I definitely knew that!”), you’re only going to hurt your learning.