What to do if you're being abused by a Low Cost company?

Normally what you describe falls under Consumer Protection laws in the country where this occurred.

Given that this is Spain you should be able to find relevant information on the European Consumer Center in Spain site.

But generally speaking you should use electronic payment methods if you want to keep track of your payments and have record of it in case things go awry.

best way is to read the terms of your contract and learn to understand them, then handle accordingly.
There's a 50 Euro fee for oversized bags, you either don't bring an oversized bag or you bring the money, it's that simple. Nothing to understand, nothing to "defend yourself against".
If you're one of those people who're prone to want to board an aircraft with tons and tons of stuff that's over the amount allowed on your ticket and then get upset when confronted with that, I can't have any sympathy with you and your ilk as you're a main reason for delays, overloaded overhead bins, diversions because of fuel shortages (yes, the airline needs to carry extra fuel for contingencies, but you wanting to take 100lbs of carryon luggage and stuff it in bins designed to take the luggage of 3-4 people rather than just you is NOT a contingency), frustration among your fellow passengers who have to sit with their luggage in their lap for hours because yours takes up the space designed to hold their bag, etc. etc. etc.

Actually I just remembered that I once noticed that one of the cheap airlines defined the maximum dimensions of the hand luggage which was 5cm smaller than the equivalent in British Airways. In this case even if the passenger has "hand-luggage approved" suitcase, there is a fair chance it will be too big for the cheap airline. This is probably what happened in the case described in the question.

How to defend yourself? I guess you need to read all the small print, but I understand it is very time consuming, especially that these things change.