Hi @Hasti_A,
Welcome to the :qiime2: forums!
Congratulations, you have stumbled into one of thsoe horrifying secrets of microbiome reserach that no one talks about (or everyone talks about).
I'd recommend reading the thread on decontamination:
You might also look at a slightly different perscective about well-to-well contamination if you ever used robotic processing in your samples
https://journals.asm.org/doi/full/10.1128/mSystems.00186-19
Best,
Justine