I don't think the process can be reversed but there is certainly a lot of leeway in how it will happen.
It was this that the Lib Dems were (weakly) campaigning on in the election - to give the people the vote whether to accept the deal or stay in the EU after the negotiations were complete.
If neither the UK nor the EU desires this break-up, then it won't happen. It all depends on what the UK government does (which realistically speaking probably does not include pulling the emergency brake on the Brexit, unfortunately).
However, it's unlikely Britain would back out of the decision. I think the best we can hope for is a Norway-like approach (EEA).
So its reversible, and if there was a change of government that wanted to reverse it I suspect the EU would go along. OTOH, it seems near certain that if a coalition is formed, it will be a Tory-DUP coalition, which won't want to reverse it (the DUP seems to want a softer Brexit, but it's not clear that the EU is interested in that.)
If a government can't be formed.(which doesn't seem implausible), then my understanding is that there is another election in fairly short order.
The EU want us to stay though, so at the moment I think a reversal would be quite painless. In a year's time, maybe not.