Your question is somewhat unclear. By "disinherit an inheritance" do you mean avoid receiving it? That would be a disclaimer, which one can do as long as they have not received the benefit of the inheritance, among other rules. To disinherit someone is to change one's will or testamentary substitute so that an heir no longer receives a bequest--if I have a fight with my son and remove him from my will, I have disinherited him. Good luck.
This is not provided as legal advice and does not create an attorney/client relationship.
It is not clear as to what you are seeking. If you are trying to "disinherit" someone from your will or trust, you will need to sign and amendment to either document disinheriting the potential heir.
If, however, you are asking how you could "disclaim" an inheritance due to you, you can do so by writing to the Executor or Trustee stating you do not want to accept any inheritance.
You should consult with an Estate Planning attorney who can provide you with the documents you would need in either event.
If you are referring to a disclaimer, please note that the disclaimer must be completed within 9 months of the date of death in order to avoid gift and/or estate tax consequences for your own estate. I suggest that you seek the advice of an attorney who specializes in estate planning in order to ensure that the disclaimer qualifies as a valid disclaimer under both state and federal laws. If you do not have an estate planning attorney, I suggest that you check the State Bar of California's website for any attorney who is certified as a specialist: http://members.calbar.ca.gov/search/ls_search.aspx. Less than 1/2 of 1% of attorneys in California are certified as specialists in estate planning.
This information is provided solely for information purposes and does not create any attorney-client relationship. You should consult an attorney for individual advice regarding your own situation.