First of all... What type of brake pads did you fit... Performance or standard type?
Did you replace just the pads or did you replace the discs to?
It has a bearing on how you treat them to bed them in...
With normal road type pads, unusual spongy feeling is totally normal when bedding brakes in and should be done properly and with time and care.
Simply stamping on them and expecting them to work is simply NOT an option. It can take up to 200 miles+ of steady driving and steady braking during that time.... Imagine the brake disc is like a mountain range, your brake pads are flat so when they come in contact, they are only making that contact on the peaks of the mountains... Normal wear will bring the surface area contact closer together as the surface area increases.
If you can't wait that long.....
On a safe quiet road.....Take the car up to speeds of 40 mph and brake with a medium pedal force then gently & slowly release the brake once the car is below 15mph until the car slows to prevent heat soak... Let it cool for a minute or two and repeat the process....
If things are improving, increase the starting speed gradually but keeping the brake pedal release speed the same.
The last 15mph of braking is where the most heat transfers through the pads so thats why you gently release the pedal.
Damage with heat soak can occur to the pads if left against the discs after heavy braking, they will start to crumble, so give them time to cool.
It usually takes about twenty minutes to get pads close to normal with this process so be patient
If it is just the same with no improvement:
Look for other faults
Did you remove the brake fluid reservior cap when winding the pistons back?
It's a simple thing that many people don't do or forget about.
When winding the pistons back the system is under a lot more back pressure compression than normal. The extra pressure on the system can find that brake fluid can leak in past the piston seals!
If fluid can leak past, air can replace it!......... So give the brake system a quick bleed on all four calipers.
What condition were the old pads in? Wasthere still enough "meat" on them, If so it could be an underlying problem with the caliper sliding pins so the caliper is only braking on one side. Take out the pins , clean them up with emerycloth and a light coating of copper grease does the job mate.
Hope this helps mate.....
Did you get an advice note with the pads? Usually they put them in the box with bedding in instructions