The syspref OPACFineNoRenewals includes the note "(leave blank to disable)." The sysprefs maxoutstanding, noissuescharge, NoIssuesChargeGuarantees, and MaxFine can all be disabled by leaving them blank, as well. It'd be helpful to include that same "(leave blank to disable)" message to make it clear.
Amending that, MaxOutstanding is *not* disabled by leaving the syspref blank. In that case, a blank is treated as a zero, meaning any fines at all will be enough to block holds.