How can 3' end polyadenylation protect cellular RNA from ribonuclease degridation?
According to my book Molecular Biology Principles of Genome Function by Craig et al, eukaryotic RNA degradation does not have an initiation, as in bacteria where pyrophoasphate hydrolase hydrolyses the 5'-triphosphate to 5'-monophosphate, and is therefore the initiatior. 

In eukaryotes the polyA-tail is shortened by a deadenylase enzyme complex which contains a 3'to 5' exonuclease called CCR4. But deadenylation of polyA-tail doesn't necessesarily need to lead to degradation since it is reversible by post-translational readenylation. 

Poly A tail protects the mRNA by not letting the nucleases reach the mRNA itself. As long as poly A tail is degraded, its just fine, since it can readenylate back again. The figure below shows the degradation of mRNA in eukaryotes, which can be either in 5' to 3' direction or 3' to 5' direction.



What you are thinking of is ubiquitination of proteins, which is a post-translational process which can mark the proteins for degradation.