Some DNases can be denatured check the protocol for the manufacturer whose product you are using. They might have included a STOP reagent.

You can utilize column-based kits to purify the RNA from DNase.

Traditionally you can do a second Phenol chloroform extraction to purify the RNA.

You should be able to inactivate DNaseI after treatment without performing phenol/chloroform extraction. A general protocol includes adding 1 μL of 50mM EDTA to the RNA sample (to protect the RNA from high temperatures) and incubating at 65 °C for 10 minutes. However, always check the protocol provided for your specific enzyme on the manufacturer’s website to ensure optimal results.

Most qiagen RNA isolation kits include options for on-column DNAse treatment: it's not a huge effort (add DNAse to column, wash, continue with protocol) so that's an option if you're desperate.

Alternatively, why not simply determine whether you need to bother?

Most mtDNA encoded genes are expressed at super-high abundance, because they're fairly critical to cell function, so even in the presence of contaminating mtDNA, the cDNA signal should still represent the overwhelming majority.

Hence: prepare some "no reverse transcriptase" control samples: take RNA, prepare cDNA as usual but just...don't include reverse transcriptase.

Now run your qPCR with these and measure the difference between Cqs for "samples with RT, and same samples without".

If it's more than ~6 cycles, then honestly: ignore the fact you have some mtDNA contamination. 6 cycles is a ~64-fold difference, and thus mtDNA contributes only ~1.5% of your measured signal at best.

Since qPCR really isn't useful for measuring changes over such small scales, a random extra variable of +/- 1.5% is unlikely to be of consequence.