If a named 401k beneficiary rejects the distribution on the death of the participant, what options are available to the administrator?

What do you mean by "reject"?
Note that in many plans, if the value of a benefit due is less than $5000, the plan will automatically make the distribution.

The beneficiary (who is the spouse)does not want the money (I assume for estate tax reasons).

Dave:
Ask the beneficiary if He/she would like to assign me as beneficiary. I'll take it.

No tax rate is as high as 100%. Can't imagine why the beneficiary would not want it. BTW, tax on a distribution from a qualified plan is deferred if rolled over into an IRA. Typically, a distribution to surviving spouse is made to the spouse, not to the estate of the deceased.

Beneficiary might consider delaying receipt, if that helps make a tax situation any better.