They don’t have standing to refuse.
Biden is firing people so doubt any fed would walk away from that salary retirement and benefits to refuse an order.
For all they know, eliminating ICE while proclaiming it to be a, “Racist Organization,” (With help from the enemedia) might be next on Biden’s EO list.
If anything, this might actually encourage a lot of them to just quit, as we’ve seen with police forces in leftist run cities nationwide after BLM and antifa rioting outcomes.