The proposed solution can be tricky if you have an exhaustive list of special characters to whitelist.
If you want to blacklist instead, disable the whitelist in startup.cs:
 services.AddIdentity<User, Role>(
                options =>
                {                        
                    options.User.AllowedUserNameCharacters = string.Empty;
                })
Then create your custom user validator
 public class UsernameValidator<TUser> : IUserValidator<TUser>
where TUser : User
{
    public Task<IdentityResult> ValidateAsync(UserManager<TUser> manager, TUser user)
    {                
        if (user.UserName.Any(x=>x ==':' || x == ';' || x == ' ' || x == ','))
        {
            return Task.FromResult(IdentityResult.Failed(new IdentityError
            {
                Code = "InvalidCharactersUsername",
                Description = "Username can not contain ':', ';', ' ' or ','"
            }));
        }
        return Task.FromResult(IdentityResult.Success);
    }        
}
Then add it to startup.cs:
 services.AddIdentity<User, Role>(
                options =>
                {
                    options.Password = new PasswordOptions
                    {
                        RequiredLength = 8,
                        RequireUppercase = true,
                        RequireNonAlphanumeric = true,
                        RequireDigit = true,
                        RequireLowercase = true
                    };
                    options.User.AllowedUserNameCharacters = string.Empty;
                }).AddUserValidator<UsernameValidator<User>>()