Which chemical or chemicals inhibit the activity of dnases during dna isolation?

EDTA inhibits the activity of DNase during DNA isolation process.

DNase is an enzyme which degrades DNA using divalent metal ions like Mg^{2+}. These metal ions act as cofactor for the proper functioning of DNase.

EDTA chelates the divalent metal ions and thus does not allow DNase to access them.

Therefore in this way DNA is prevented from degradation during its isolation.