What affirmation would you with Isa that wouldn't just completely desensitize someone?
With proper care, Isa won't harm you any more than Thurisaz, Nauthiz, Uruz, or other "dangerous" runes.
You can mix Isa with another rune that gives protective benefits, with the intention of keeping Isa working positively. This, combined with positive intent and focus on the positivity of your work should be sufficient.
I have completed a 40 day working of using Isa and Berkano for enhancing my ability to calm and focus my thoughts. I have had no negative issues from this. It gave my head a degree of silence, and it now feels like my thoughts have more "weight" to them, in a good way. My mercurial abilities have not been damaged, nor have any physical changes.
In this case, Berkano both added a degree of silence or calm, like the moon, to help my working, and added protection from any negatives of Isa. Nauthiz, for example, is similar to ISA in some ways and can be combined with it for a similar purpose.
Personally, I want to say that Isa can be used on its own just fine, assuming proper programming precautions. Yet, I followed the above options due to HPS Pythia's advising caution to be used. I think someone could hurt themselves badly with plenty of other energies, not just the one that represents Ice.