Payment problems aren't unusual, and are often simply that the guest made a mistake.
But multiple payment problems are a red flag to me. Even if they manage to sort it eventually, it suggests there's something amiss, and there's likely to be other things not entirely right with that guest.
And yes, Airbnb will almost always side with the guest even if the guest is totally at fault, so both comments above are very good examples and advice.
So, it's always better to find a benign way to decline the booking when the flags go up.
When a guest doesn't qualify for IB, I tend to ask a lot of questions before allowing them to book. It's not a guarantee, it's tedious, I don't like doing it, and it often scares them away. But just one bad guest can cost more than the next 10 good ones. Watch your back!