When you sign up for Twitter, your tweets are public by default; anyone can follow you, view your tweets, and interact with you. To prevent strangers from following you, set your Twitter account to private. That way, you receive a request when new people want to follow you, and you can approve or deny the request. Here's how to set your Twitter account to private.
Open Twitter and select the Settings (gear) icon.
Select Privacy and Safety.
Toggle on Protect your Tweets.
Select Done.
Your tweets will appear only to people who follow you, and if anyone wants to follow you, you have to approve them.
To approve or deny follower requests, go your Twitter profile. Select Followers > Pending follower requests to view the people who have requested to follow you. Select Accept or Decline.
How to Remove a Twitter Follower
If you already have a stranger who follows you, your first course of action is removing the follower, which you can do from the web. Here's how.
On the web version of Twitter, click Profile.
Click Followers.
Click the three-dot menu next to the follower you want to remove.
Select Remove this follower.
Followers you remove this way won't receive a notification that you've done so, but they can still see your feed if they go to your profile.
Block a Twitter Follower
Removing a follower doesn't stop them from refollowing you later if you set your account to public. In this case, you need to take stronger measures. If a stranger (or someone you know) is following you on Twitter and you want to block them:
Select the down arrow icon located at the top of a Tweet from the account you wish to block.
Select Block.
Select Block again to confirm. Now the blocked account can't follow you or view your tweets.
Use a service such as StatusPeople's Fake Follower Check to see what percentage of your followers are fake, real, or inactive.
Why Are Strangers Following You?
There are many reasons for someone you don't know to follow you on Twitter. They may admire and connect with your wit, point of view, and humor, or they may have you confused with someone else.
Sometimes a random person might follow you in order to get a follow in return. This is a common practice for people who want to build up their followers. More followers means more visibility, whether you just want attention or genuinely have something to promote.
Not all random people have innocuous reasons for following you on Twitter. Hackers and internet criminals may send out malicious Twitter bots to follow you. Malicious bots spread links to malware. These links are often disguised as shortened links so that the dangerous link is obscured from view.
Before you click on a random short link on Twitter, use a link expansion service to inspect it so you can decide whether its destination is somewhere you really want to go.
Random followers might also be spammers, who use every possible avenue, including Twitter feeds, to spread their messages. Spammers follow a massive number of accounts hoping for follow-backs, increasing their audience.
If you report a tweet as spam, Twitter blocks the user from following you or replying to you but doesn't automatically suspend the account.
It's easy to report spammers to Twitter. Select Followers from your Twitter home page, and then select the button to the left of the Follow button and choose Report.