IRC 401(a)(13) also addresses this. Here is the first part of that subsection:
(13) Assignment and alienation.--
(A) In general.--A trust shall not constitute a qualified trust under this section unless the plan of which such trust is a part provides that benefits provided under the plan may not be assigned or alienated. For purposes of the preceding sentence, there shall not be taken into account any voluntary and revocable assignment of not to exceed 10 percent of any benefit payment made by any participant who is receiving benefits under the plan unless the assignment or alienation is made for purposes of defraying plan administration costs. For purposes of this paragraph a loan made to a participant or beneficiary shall not be treated as an assignment or alienation if such loan is secured by the participant's accrued nonforfeitable benefit and
is exempt from the tax imposed by section 4975 (relating to tax on prohibited transactions) by reason of section 4975(d)(1). This paragraph shall take effect on January 1, 1976 and shall not apply to assignments which were irrevocable on September 2, 1974.
There are some exceptions, such as QDRO's.
IRC sections relevant to employee benefits can be accessed here:
http://www.benefitslink.com/taxcode/index.shtml