I have made a git pull request with my repo. Sometime later, before pull request got approved, I proceeded to make another commit&push which also got pushed up to pull request.
Is there a way to remove the last commit from pull request and how to prevent this from happening in the future?
Yes, you simply can reset your branch to the previous commit, and force push: the pull request will be automatically updated.
git checkout yourBranch
git reset --hard yourBranch~
git push --force origin yourBranch
Then, if you want to make such an error harder, delete your branch locally: you won't checkout it or use it by mistake.