How can a continuous RNA be transcribed in the lac operon?
RNA Pol doesn't worry about stop codons.  Transcription termination can occur through the formation of a hairpin in the new RNA sequence, or through the action of Rho proteins.  A lot of the time prokaryotes have polycistronic mRNAs, that is, mRNAs with multiple protein coding regions.  The stop codons are detected during translation, so you will often have all three lac proteins being translated at once off of the same mRNA.

Here's a nice section on transcription termination, or look up Rho-independent or Rho-dependent transcription termination for more info.