I think kash is quite right here, except the second point..
the limit of number of wins is already there - requiring the uncertanity is exactly this.. if this isn't enough to discourage them few more wins against AI will hardly do.
the activity isn't good idea as well, I think... for those who only want to play from time to time.. and again, it wouldn't be problem to fake it..
as I see it, solution for the 2nd problem should be somewhere in IP check - and then only 1 account can participate in ladder from the IP.. you should claim which one and this cannot be changed (exceptions might be given by admins) the other occounts would be allowed to play games, play even ladder games - for others to gain points if the win against it - but just simply wouldn't gain any points by itself..

About the IP check i dont really know how it workd but my question is this. If ¿I and my brother play in the same PC then would we have problems?

Taht is not true to this game since my brother has his own PC now but it have been true to other games for a long time

Nayen's question shows the problem with IP based filtering: people sharing a residential internet connection will typically share an IP.

An activity rule would certainly be possible to be faked. Realistically, you will never totally eliminate people attempting to game the ladder as long as there are still rewards. Even without rewards, some people may be willing to try. The balancing act is to find a rule that reduces the amount of abuse to an acceptable level while not unduly burdening normal players.

I still think gimping the use of phantoms and defaults is a good way to go.

Players cannot make new accounts very quickly to beat new players and have multiple free accounts working their way up the ladder. By reducing the number of points players earn while using free/phantom cards you encourage the use of paid cards.

Are people actually just using phantoms and defaults? I had assumed they were transferring cards over from their main account and then transferring them back after they finish with the ladder.

A simple solution could be> make MORE ladders.
For example:
Ladder 1 for 1000 gold, for players above 1400 rating (or 1300?) without any phantom cards (20 prizes: avg 50)
Ladder 2 for 400 gold, for lower rating players and with phantoms (20 prizes: avg 20)
Ladder 3 for 100 gold, for newbies, below 30 (or 40) wins, using only default decks. (10 prizes: avg 10)

IP check should be used, and only 1 ladder prize awarded for the accounts from the same IP address. And so, it would be a worthless effort to participate in all ladders.
Yes, it could cause some problems (for example, my son uses the same computer for the same game but I won't allow him to play my account, so he has to play his own), but a few e-mails for the admins could solve the problem (exceptions based on log and behavior...)

When the ladders start, you are automatically assigned to a ladder (or you have to join a ladder, like leagues?) and you can play similar games only against players in the same ladder.
Next to the user name it could be stated which ladder is the player participating in.

I hope that discussions will improve my idea and we'll find the good solution together.

^^ that I think is a step in the right direction.

What about instead of having a separate system of ladder points, you can measure how much a person's rating goes up in a week? Is that too abusable? The only worry there is people purposely losing to their friends over and over again and skyrocketing ratings but that will have quite diminished returns.. and yes, this screws people who have high ratings but all versions of the ladder system so far already do that, which is why people start new accounts etc..

I think you could help fix the ladder by rewarding players with high ratings with something, right now there is no incentive to have a high rating and it is actually worse because there are less people to play similar battles against. Then people wouldn't just tank their ratings to play in ladder.

My idea is to have a tournament that only 1500+ or maybe 1400+ could enter once a week.

Idea of Ladder is to give a chance to new players to gain some gold - so those with high ranking aren't considered to be new players, as they had to make up for their high ranking...
(but idea of a tourney or whatever for 1500+ players sounds good ^^)