I reproduced the error in a bare minimum form + signature and neither setting the form nor the input to disabled prevents the user from drawing in the box.
Even when disabled, the user can draw and the value is updated.
My only idea for a workaround is to capture the current value of the signature field, and have the new value get tossed and replaced with the old onChange.
Hey @khill-fbmc, I tried to replicate the behavior, It lets me draw in grey right after changing the disabled attribute for either the form or the input. But once you go into preview mode it works as intended.
Are you still seeing the same behavior after refreshing app state?
Even in preview mode, I can draw in light grey, "unlock" my form and press save and it updates.
Shouldn't the intended behavior be that when "disabled" it rejects user input? the value property updates still.
I made a minimal reproduction here
Hi @khill-fbmc, thank you for flagging this issue and for taking the time to build the public app for us to see exactly what is going on with all details, I was easily able to reproduce it.
We created a bug report for it. I will keep you posted with any updates from the Engineering Team.
The only workaround I came up with is to toggle the "Hidden" property for the "Signature Component" instead of the "Disabled" property. I hope this temporary work around works for you, but I completely understand this is not the best approach in terms of UX.
Hey @khill-fbmc, great news!
The PR for this bug fix has been scheduled for the next Cloud release (3.39.0), it should come out next Thursday. 
Great news! Thanks for the update