The best way to protect your Twitter account from spam is to get rid of all the existing spam/fake followers you have in your account because a spam follower will attract more spam accounts to follow you.
Focus on the hashtags that you are using, focus on the main and important keyword, avoid using spammy hashtags.
You can get rid of the spam/fake followers present in your account with the help of FollowerAudit.
Followeraudit is a Twitter fake followers checker tool that can audit your followers and list down all fake and inactive followers separately.
You can remove those fake followers from your account.