You could try using string replace. e.g.

var name = "! net-tech-"
// replace ! with "" and replace the space with ""
var newName = name.replace("!","").replace(" ","")

newName should give you "net-tech-"

Alternatively, you could use an if statement to check if the name starts with "!", e.g.

if (name[0] === "!") {
    // get the string from 2nd character onwards
    var newName = name.slice(1)
}

Fetch The first step would be to fetch all the members, which you've done so far.

Filter Next filter all members with usernames that have ! or any extra spaces.

If you want to replace only the ! if they are at the start you would use String#startsWith(), for any ! in the username use String#includes().

To remove all extra spaces you can use the Regular Expression / +/g

Traverse Loop through the resulting members and change their nicknames

const targetChar = '!';
const extraSpaces = / +/g;

message.guild.members.fetch().then(fetchedMembers => {
   // Use startsWith() for only starting "!", Use includes for any "!"
   const membersToRename = Array.from(fetchedMembers.filter(m => m.user.username.includes(targetChar) || m.user.username.match(extraSpaces).length));

   if (!membersToRename.length) return;

   // Use replace() for only the first, replaceAll() for all
   membersToRename.forEach(m => {
      const newNickname = m.user.username
         .replaceAll(targetChar, '')
         .replaceAll(extraSpaces, '');
      m.setNickname(newNickname)
         .catch(console.error);
   });
});

Warning This could be considered API spam since you're changing information in masses, especially if used in large servers.