Yes.  You may stop making contributions to a 401(k) plan by notifying the plan administrator.  However, any amounts you have already contributed to the plan must remain in the plan until one of the conditions permitting distribution occurs.  Those conditions are death, disability, normal retirement,  separation from service and in some plans, the event of specific hardships. Most plans have designated specific times at which changes may be made.  Generally, you may make changes in your deferral percentages at the beginning of each plan quarter.

10/17/2008