To disable the sharing link or limit sharing to the current list members, you just need to select the sharing icon to open the sharing menu. Then, click or tap More Options. Here you can choose to Limit access to current members and no additional members will be able to join the list.
Alternatively, you can select the red Stop Sharing button to turn off the sharing link. Once you stop sharing, current list members will no longer have access and your list will be private again.