def pre_request(worker, req):
    if req.remote_addr != 'ip to be blocked':
        pass

from flask import request, abort, current_app as app

ip_ban_list = ['127.0.0.1']

@app.before_request
def block_method()
    ip = request.environ.get('REMOTE_ADDR')
    if ip in ip_ban_list:
        abort(403)