Of course Baptism cannot be undone.
Many things in a person’s life happens and cannot be undone as well.
Let’s say I was born in Venezuela. The circumstances of my birth CANNOT be undone. But I can undo my being Venezuelan by becoming the citizen of another country.
So, what about church membership? Is one considered Catholic or Anglican or Methodist, for life by virtue of Baptism (even when one no longer subscribes to the tenets of the faith?).
Don’t confuse cirumstance with ontology.
Don’t confuse circumstance with ontology.