I think it's better for you first used Chloramphenicol than Ampicillin in SABOURAUD DEXTROSE AGAR ( with 0.05 g Chloramphenicol that dissolves chloramphenicol in 10.0 mL of 95 percent ethanol for 1.0 liter) and the second removes the yeasts on PDA.
Article Screening Wild Yeast Strains for Alcohol Fermentation from V...
The objective was to isolate yeast from fruits, but it should probably also work for you. Shortly use combination of chloramphenicol, dextrose and lysozyme. In materials and methods you have a detailed protocol.