just FYI, there is no risk in connecting to any of those servers with your "real" Electrum wallet. they are in fact Electrum servers and they can't do anything to harm you. the only thing they do is that when you send a transaction they ask you to "click a link" and "download their malicious software". so don't click that link!

other than that, as long as you know this, you don't have to worry about what server you connect to.

but if you are so worried about it, you can not blacklist a server but you can force Electrum to always connect to one server. go to your network window and switch to network tab, deselect automatic connection and choose a server to connect to manually.