Linux – How to get rid of this vertical line in Geany

No, Geany is not able to do this and most likely will not unless somebody is adding a plugin for, hacking Scintilla default behavior.

I'm trying to figure out where the comment block style is defined in Geany for C files.

All color definitions and other filetype specific settings are stored in the filetype definition files. Those settings are colors for syntax highlighting, general settings like comment characters or word delimiter characters as well as compiler and linker settings.

...

Comment_single

• A character or string which is used to comment code. If you want to use multiline comments only, don't set this but rather comment_open and comment_close.

• Single-line comments are used in priority over multiline comments to comment a line, e.g. with the Comment/Uncomment line command.

  Example: comment_single=//

comment_open

• A character or string which is used to comment code. You need to also set comment_close to really use multiline comments. If you want to use single-line comments, prefer setting comment_single.

• Multiline comments are used in priority over single-line comments to comment a block, e.g. template comments.

  Example: comment_open=/*

comment_close

• If multiline comments are used, this is the character or string to close the comment.

  Example: comment_close=*/

comment_use_indent

• Set this to false if a comment character or string should start at column 0 of a line. If set to true it uses any indentation of the line.

  Note: Comment indentation

  comment_use_indent=true would generate this if a line is commented (e.g. with Ctrl-D):

  #command_example();

• comment_use_indent=false would generate this if a line is commented (e.g. with Ctrl-D):

  # command_example();

• Note: This setting only works for single line comments (like '//', '#' or ';').

  Example: comment_use_indent=true

Source Filetype definition files

I would still be interested in a way to have the comments added to the beginning of the line rather than at the indent level.

Use comment_use_indent=false