TL;DR

Fake leads come from four places: automated bots, click farms filling forms by hand, competitors draining your ad budget, and real people your targeting should never have reached. Each needs a different fix, and adding a captcha only helps with the first one.

Work out which kind you have

The fix depends entirely on the source, and most people guess wrong because everything arrives looking similar in an inbox.

What you seeLikely source
Gibberish, random links, nonsense namesAutomated bots
Plausible details, real-looking names, never answer the phoneClick farm
Spikes from locations you do not serveClick farm or targeting
Well-written pitches selling you SEO or web designA person, deliberately
Real people asking about things you do not sellTargeting or messaging
Repetitive identical submissionsScripted
High volume at odd hoursAutomated
Started suddenly when you launched a campaignAd-side

That last row is the one worth checking first, because the timing tells you more than the content does.

The four sources

Bots

Scripts crawling for forms to submit, usually to place links or test addresses. High volume, low quality, easy to spot and easy to stop. A honeypot and a timing check handle most of it, free and invisibly.

Click farms

People, paid a small amount per action, clicking ads and filling forms by hand. They produce plausible-looking enquiries with contact details that go nowhere. They pass every captcha, because they are human.

The signals are volume-shaped rather than content-shaped: unnatural spikes from irrelevant locations, repetitive submissions, and high rates at hours that do not match your market.

Competitors

Clicking your ads and filling your forms to drain budget. Rare, but it happens, and it looks like a click farm from the inside.

Your own targeting

The largest source on most accounts, and the one nobody wants to hear. These are real people who genuinely filled in your form and are of no use to you, because the traffic was bought too broadly.

Common causes:

CauseWhat it does
Search PartnersPlaces ads on sites with much lower intent. Turning it off commonly improves lead quality immediately
Location targetingGoogle’s default targets people in or interested in your locations, which includes anyone searching about your area from anywhere
Broad match with thin negativesBuys searches with no purchase intent
Missing negative keywords“Free”, “jobs”, “salary”, “template” and similar bring people who will never buy
Display NetworkIncluded automatically on some campaign types

Google describes the broad setting as presence or interest and the narrow one as presence, and recommends the broad one as a search best practice, which is why so many accounts are running it without having chosen it.

None of that is spam in the technical sense. It is real people you paid to reach and should not have.

The tell that it is ad-side

If your conversion rate looks unusually good while the phone does not ring, the problem is almost certainly where the traffic is coming from rather than your form.

Turning on captcha protection typically does not shift it, because the traffic is human. Disabling Search Partners often does, and it costs nothing to test.

Fix in this order

1. Check the ad settings. Search Partners off, location targeting set to presence only, negatives in place, Display separated. Free, and it addresses the largest category.

2. Add the free form defences. A honeypot and a timing check. Minutes to add, invisible to visitors, stops the automated share. The full list is in how to stop form spam.

3. Add filtering that reads the submission. Content and reputation scoring is the only thing that catches spam a person typed.

4. Stop feeding the loop. If you count form submissions as conversions, the fake ones are training your ad platform to find more of the same traffic. That loop is covered here.

Fake leads distort your channel numbers

Fake leads waste sales time, which is the obvious part.

The part that costs more is what they do to your reporting. Every fake enquiry is counted as a lead and attached to whatever brought that visit in, so the channel producing the most junk shows the lowest cost per lead and attracts more budget. What spam leads do to your marketing numbers works through that in full.

Lead Source records the source on every enquiry whatever form you use, so you can at least see which channel the junk is arriving on. Stopping it at the form is a separate decision, and it means using a Lead Source form rather than adding anything to your current one. See how form spam protection works, or how the source on each lead is captured in the first place.