The subject invention concerns compositions and methods for blocking cancer cell growth or proliferation and/or inducing cancer cell death. Compositions of the present invention are peptidomimetics that inhibit STAT function. Peptidomimetics of the invention include compounds of the formula RY*L (where Y* represents phosphotyrosine), with the R group at the Y-1 position. Peptidomimetics of the invention disrupt Stat3 activation and function. Peptidomimetics of the invention significantly inhibit tumor cell growth and induce tumor cell death.