blocking a dna adapter / linker against 5->3 fill in ?

[Ralf Sigmund]

Hi,
I would like to digest cDNA and afterwards ligate it to two different
adapters.
one should look like this:
5'-GNNNNNNNNNNNNNNNNNNACGATGAGTCCTGAG-3';        top linker

3'-TACTCAGGACTCGC-5'   bottom linker
                                                                  !

Now I would like to block the 3'-terminal base of the bottom linker, so that
Taq-Polymerase cannot fill in the 5'-3' direction.
I thought about ordering a oligonucleotide with a 2',3'-didesoxy group in
this position. However this modifaction appears to be very expensive.
Is there a way to block an normal oligo enzymatically ?
TIA
Ralf