Can we cancel out a kamma by doing a strong opposite kamma? Can a bad kamma be cancelled out by doing a strong good kamma?

If the answer is no, at-least can we reduce the effect of that bad kamma by doing so?

If the answers are no for the above two questions, can we postpone the time which kamma gives its result by doing a strong opposite kamma?
Bad kamma once sown cannot be erased completely, but by cultivating Sila, Samadhi, Panna, the suttas say it's possible to lessen the intensity/severity of it. See the case of Ven. Angulimala in MN 86 and the Buddha's famous quote:


  Bear with it, brahman! Bear with it! The fruit of the kamma that would have burned you in hell for many years, many hundreds of years, many thousands of years, you are now experiencing in the here-&amp;-now.


Also see the Lonaphala Sutta - AN 3.99