Short answer

Settings > Marketing > Forms > Submissions Settings has account-wide controls: mark submissions from excluded IPs or referrers as spam, Gibberish detection for random strings, and Block email domains across all forms. Visitors with a blocked email address or domain cannot submit. Blocking specific domains or free email providers needs Marketing Hub or Content Hub Starter or higher. Individual forms can also block free email providers.

1. The settings

In my demo portal, excluded IP submissions are marked as spam, gibberish detection (Beta) is off, and no domains are blocked yet.

Forms Submissions Settings tab with switches for excluded IP spam marking and gibberish detection and a Block email domains across all forms field — Excluded IP spam marking (1), gibberish detection (2), and blocked domains (3).

2. Which tool for which spam

Junk text in fields: gibberish detection. Property rules also let a field opt in, as seen in property validation rules.
Disposable or competitor addresses: blocked domains.
Your own team testing: exclude office IPs. See excluding your own traffic.

3. The opposite problem

If real submissions are being marked as spam, see form submissions marked as spam.

Sources

HubSpot Knowledge Base: Prevent and filter spam in form submissions
HubSpot Knowledge Base: Block form submissions from specific email domains

Cut HubSpot form spam: blocked domains, gibberish detection and IP exclusions

Short answer

1. The settings

2. Which tool for which spam

3. The opposite problem

Sources

New field notes by email

Still stuck?