Yeah, a small nail will work. You really don't need to push it all the way through. Just enough to get the sear and spring free.
Replace the spring and push it back. If it seems loose, just use the pointy end of the nail and "gently" tap it inside the hole of the roll pin to spread the roll pin just a little on both ends.
Why do you want a harder sear spring? Did you do some "evil" file work on the back of your bolt to get a hair trigger?
