Don't use .htaccess to deny access to your website. Check out something like CSF, APF, Fail2Ban, straight IPTables, etc.
What is the ISP that you are trying to block, and for what reason? Knowing this may result in you getting a better answer to your question.
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I know .htaccess will work but the reason you gave about delay is that what i was thinking. It might create delay problem. So any other recommended method like from iptable? if so how I can banned host from iptable?
iptables deals with IPs, not hostnames. If you use hostnames, each time a packet comes in (this includes the requests you're doing from server to internet, as the "internet" sends you packets back) it will have to resolve the hostname, which means your server will be unusable.
so, if you wish to block the *.uk.vpn.com, each time you see the spammer, get the IP and put it in a firewall... sooner or later you'll get all the IPs.
another better idea: try to identify the spammer by browser, nickname, something unique that he's using and block that. I guess it will be easier than to block *.*.vpn.com...