The only suitable punishment has to be that Southampton CANNOT be allowed to be promoted. It's really that simple.
Fines are meaningless as the reward outweighs any fine that could be levied. Plus if they stay up after their first season they continue to reap the windfalls from that.
Transfer embargo or points deduction don't have to be applied by the PL so again more or less toothless until such time as they get relegated back to Champo.
And if they aren't thrown out, so either Burra play in the final due to receiving a bye or Southampton play Hull in the final and if they win are expelled afterwards, the message has to be that this type of behaviour results in the strongest possible sanctions. If not EVERYONE will be at it next season.
During the summer additional investigations should add the points deduction, transfer embargo and/or relegation to L1 or even L2, IF it's proved that their miraculous upturn in form was directly linked to more cheating, sorry spying.