You unfortunately can’t prevent them, it’s very problematic, hopefully Roblox will fix this, it’s called a ddos attack. The only current method you can do is act that you don’t know that’s it’s an exploiter and say that you fill fix the bug to the exploiter, so he gets bored and stops.
The exploiters are usually able to crash the servers by spamming a remote that creates or physically affects some part/model on the server.
What you can do are sanity checks (how often does the client send the remotes?) and fake replication if possible (server does not create a part, it sends a remote to all the clients and they are creating the part, so the server is not overloaded).
I don’t think it would lag clients.
A really simple fix is to have like a checker every x amount of seconds to see how many times a player has fired the remote, if it’s a really big amount it means they’re trying to crash your server and you can just kick them so they stop firing.
Unfortunately this shouldn’t fix DDOS attacks but will allow you to kick crasher that are attempting to crash inside the game