Any process in which a virus stops, prevents, or reduces the frequency, rate or extent of host ubiquitin-like protein ISG15 activity. ISG15 is a ubiquitin-like protein that is conjugated to lysine residues on various target proteins. Viruses escape from the antiviral activity of ISG15 by using different mechanisms; the influenza B virus NS1 protein for instance blocks the covalent linkage of ISG15 to its target proteins by directly interacting with ISG15. The papain-like protease from the coronavirus cleaves ISG15 derivatives.