Even a pardon won't allow you to cross anymore.

Fizz wrote:
We've been crossing the border at least twice a year for 45 years.
Never, ever had a problem or even a hint of a problem, so... it must be you.

The problem is the entire border crossing is up to the discretion of the border agent (both directions, both countries) and you have NO rights at the border for search procedures (again both countries). No reason required either for denial of entry.

The likelihood is small for most people, but it CAN happen to anyone.