Use notepad++'s replace feature. It accepts regex expressions and you can just replace it with nothing.
"STEAM_0:(0|1):\d+"\s\n{\n\s
matches a steamid followed by the first brace and some spaces and newlines
"deny"\s\n\s{\n\s\}\n\s
matches "deny" followed by some spaces, new lines, and braces
"allow"\s\n\s{\n\s\}\n\s
matches "allow" followed by some spaces, new lines, and braces
"name"\s".+\n\s
matches "name" and someones name, followed by some spaces and new lines
"group"\s"new"\n\}\n
matches "group" and "new", followed by some spaces and new lines and the closing brace and then another newline
all put together:
"STEAM_0:(0|1):\d+"\s\n{\n\s"deny"\s\n\s{\n\s\}\n\s"allow"\s\n\s{\n\s\}\n\s"name"\s".+\n\s"group"\s".+"\n\}\n
Of course because of the genius of users.txt, the deny, allow, and name fields can be in any order. Hip hurray! (I'm sure there must be a good reason for this, and I'm curious what that reason is. It's just also pretty irritating.)
Just go ahead and re-order those three elements (total of six possible combinations) and you should be able to get them all.