Provided herein are compositions, systems, methods, and kits for joining together the ends of one or more polynucleotides using at least one pair of blocking oligonucleotide adaptors. Blocking oligonucleotide adaptors can be used to reduce the formation of adaptor dimers or trimers (or higher-order concatemers) which can improve the yield of desirable polynucleotide-adaptor products in any recombinant nucleic acid workflow. Blocking oligonucleotide adaptors can comprise a double-stranded oligonucleotide adaptor (duplex) having an overhang cohesive portion that anneals with a blocking oligonucleotide which can be a separate single-stranded oligonucleotide. A blocking oligonucleotide, when annealed to an overhang portion, can prevent undesirable hybridization of the overhang portion to another nucleic acid, such as the overhang portion from another blocking oligonucleotide adaptor or a polynucleotide of interest.