If the house is owned as joint tenants, transfer it to yourselves as tenants in common so that when you die your interest will go into your estate rather than directly to your spouse.
Have a properly prepared and executed will leave everything to whomever you like, with a life interest in the house and annual allowance in trust for your spouse.
Be certain to have your will state that you do not want anything to go to the step son.
Have your sound mind at the time of the making of your will, and you opposition to your step son, attested to by several people who know you well, and keep their affidavits with the will.
This will make it difficult for him to collect or for him to get in good again with your spouse and collect your assets via her estate when she passes.
Depending on where you live, there may be laws which permit immediate family to ignore the will and take a large percentage of the estate despite your wishes. To get around this, bulk up on insurance with named beneficiaries so that the insurance pay out on your death does not go through your estate. Pick insurance policies which are also annuitites which supply you with a retirement allowance each year.
Of course the best was to avoid leaving the step son anything is to spend it all while your a still alive, and have fun doing so.
You should also thnk about what would happen if you did not die but had tobe placed in care.
To avoid your step son making a run at obtaining guardianship, you should have properly drafted and executed powers of attorney for both personal care and for property which specify who will be your guardian, and which also specify that you step son should not be your guardian. Again have your sound mind attested to, and keep these documents with your will.
Finally, once the will and powers of attorney are done, gather everyone who might possibly be interested in you or your setate, includng your step son, and your lawyer, and publicly read the will to them.