Phosphorylation is the most common reversible post-translational modification (PTM) of proteins. Because a given kinase often has many substrates in a cell and is involved in numerous functions, traditional inhibition of the enzyme leads to unintended consequences. Here we report synthetic receptors to manipulate kinase phosphorylation precisely for the first time, utilizing the receptors' abilities to bind peptides with high affinity and specificity. The inhibition enables selective phosphorylation of peptides with identical consensus motifs in a mixture. A particular phosphosite can be inhibited while other sites in the same substrate undergo phosphorylation. The receptors may work either individually on their targeted strands or in concert to protect segments of a long sequence. The binding-derived inhibition is able to compete with protein-protein interactions within a multidomain kinase, enabling controlled PTM to be performed in a previously unavailable manner.