It was recently brought to my attention that for now, you can cancel lookups with the LFForm object
 

// When lookup rule 2 is triggered, 
// if step is "Start", 
// cancel the lookup call.

LFForm.onLookupTrigger(() =>  {
    if (LFForm.step.name !== "Start") {
        return { cancelLookup: true };
    }
}, { lookupRuleId: 2 }); 

It has a slightly odd behavior if you are using a single line field with autosuggestions to trigger lookups, but works fine otherwise.

But, the team is doing a refactor of our lookup rules right now so we can better maintain existing and future use cases. Adding conditions to lookups will come after we finish this effort!