This is more of a technical than a visual issue involving sprites or NPCs or anything like that.
What a string needs to display on-screen on demand without a script is the following:
- An Interior or Dungeon style Dmap.
- A Guy.
- A String.
Your best bet is to make the actual tiles of one of the "Guys" empty in the tiles pages. Those are usually hard-coded on Tiles Page 0. Most folks usually use the "Fire" NPC, though do make a backup of the tiles first.
Here's a list of the Guys and what tiles they use.
If it's still happening, try changing the Dmap type. Though that's the sort of thing you'd start with. Dmap types are serious business, and shouldn't be changed without real considerations.
Where is this string going to be, exactly?