Eukaryotes quickly degrade aberrant transcripts containing premature stop codons using a system called nonsense-mediate decay (NMD). However, it's my understanding that the system requires the presence of an exon splice junction downstream of the premature stop codon. I.e. the failure of the ribosome to remove exon splice junction proteins from the mRNA transcript due to a premature stop codon upstream of exon splice junction is sensed by up-frameshift proteins (UPF) and the mRNA is degraded. Good diagram of this here: https://en.wikipedia.org/wiki/Nonsense-mediated_decay

That all makes sense, but how is the process controlled when there is a premature stop codon in a gene lacking introns (and hence exon splice junctions)?