Unfortunately, websites that may offer a free sign-up option for a public listing or profile are susceptible to unwanted sign-ups, this is quite common on the internet and usually comes from SEO companies trying to get their clients links on other sites.
Here are a few recommendations to address this:
1) Navigate to Members >> Search Members and identify the IP Joined from the unwanted listing. Block the IP and delete the listings created by that IP. This step usually gets rid of the unwanted sign-ups:
2) Require admin moderation for all free sign-ups under the Finance >> Membership Plans >> Edit view:
More information about this process can be found in the article down below:
Manually Approve New Member Signups
3) Do not allow free members to post links via its membership plan settings, which removes the incentive for spam signups:
4) Charge $1 to join the basic level. This will stop all free sign ups since now there is no free sign-up option and it's required to pay a nominal $1 to join the site with a listing.
You may choose to utilize all or some of these strategies. They will assist with combating unwanted sign ups from joining a site with a free membership option.