This is not a simple matter. Most people are on dynamic IPs which means that you most likely won’t be able to block an individual based on their IP since their IP will likely change each time they come to your site. You might be able to block certain regions or countries based on IPs, but that falls apart if someone is using a VPN.
If as you say, " I want only specific members to get access to my site", why not just have them log in? The login process is simple, secure, and robust.