Phenolic compounds recovered from olive mill wastewater (OMWW) represent a promising resource for sustainable antimicrobial applications. While their antioxidant properties are well documented, limited data are available on their antimicrobial activity against spore-forming bacteria such as Bacillus cereus. This study assessed the antimicrobial efficacy of a purified extract of OMWW submitted to a spray-dry process on a support of maltodextrin DE-19 against three B. cereus strains (one field and two reference). Microtiter-based assays were used to evaluate bacteriostatic and bactericidal effects on vegetative cells and spores at 20 °C and 30 °C. Minimum Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC), and Killing Quotient (KQ) were determined. At 20 °C, vegetative cells were inhibited at 0.095–0.187 mg/mL, with one strain highly susceptible. At 30 °C, MICs ranged from 0.187 to 1.5 mg/mL. Bactericidal activity was observed at 0.75–3 mg/mL. Spore germination was inhibited above 0.75 mg/mL, though no sporicidal effect was detected. In general, the experimental setup using phenolic extracts can present significant challenges due to their chemical complexity and variability. Nonetheless, this phase is essential to define appropriate dosages and mechanisms of action prior to any application in food matrices. Here, the stability of the spray-dry extract allowed for the proper build-up of in vitro tests. The results highlight the potential of OMWW phenolics as natural antimicrobials, particularly effective against vegetative forms of B. cereus.