If for whatever reason an employer wants to no longer allow elective deferrals in their Plan, does a plan amendment have to be signed before those deferrals are no longer taken from an employee's compensation? Or would a notice to the employees telling them deferrals will no longer be accepted suffice, with a plan amendment signed by the end of the plan year.

Before deferrals stop, there has to be a plan amendment removing that particular Article/Clause from the Adoption Agreement, including a date certain as to when deferrals will no longer be allowed.

The Summary Plan Description has to be modified and supplied to all eligible participants.

One can't just send out a Notice saying deferrals will cease on "x" date because that is in conflict with the Plan provisions.

Well, almost. You do not have to modify and re-issue the SPD. An SMM should do it.

Also, the revised SPD or SMM need not be distributed before the elective deferrals may be discontinued.