The most reliable way to remove contaminating genomic DNA from RNA extracted with the QIAamp RNA Blood Mini Kit is to perform a DNase digestion step. You can use either:

• RNase-free DNase I treatment on-column (QIAGEN offers the RNase-Free DNase Set as an add-on, which integrates directly into the kit protocol), or
• Post-extraction DNase digestion (e.g., TURBO DNA-free Kit, Invitrogen; or RQ1 RNase-Free DNase, Promega) followed by heat inactivation or cleanup with spin columns.

Including DNase treatment is strongly recommended when RNA will be used for downstream applications like RT-qPCR, since even small DNA contamination can affect results.