For tax assessments, the Anti-Alienation provision does not prevent an IRS levy. Here is the relevant portion of the blog entry (at the end):
However, keep in mind that, in criminal tax cases, the restitution that is awarded to the IRS either by contract (i.e., the plea agreement) or by the court for tax-related Title 18 counts (such as conspiracy under Title 18 USC Section 371) is restitution for a tax liability that will be assessed as a tax. And, even where restitution for the tax is not awarded, the IRS will likely move to assess the tax at issue in a criminal tax case. As a tax, the IRS can collect from the retirement account even if otherwise protected by the Anti-Alienation provision. See 26 USC § 6334 - Property exempt from levy, here. Subsection (a) lists items exempt from a tax levy, but only exempts "certain" retirement plans as follows (emphasis supplied).
(6) Certain annuity and pension payments
Annuity or pension payments under the Railroad Retirement Act, benefits under the Railroad Unemployment Insurance Act, special pension payments received by a person whose name has been entered on the Army, Navy, Air Force, and Coast Guard Medal of Honor roll (38 U.S.C. 1562), and annuities based on retired or retainer pay under chapter 73 of title 10 of the United States Code.