To remove it, try this: 1. Open the Properties panel through the Panels button located at the bottom right of the workspace. 2. Click outside of the board and the Properties panel should display the Board information. 3. Scroll down to the Guide Manager, and a single entry of a work guide will be visible. 4. Either disable it by unchecking the "Enabled" checkbox or delete it completely by using the trashcan icon.
Note: Guide lines (and grid lines) will only show if the box is checked (default) for "Show Grid" in the General Settings region on the View Options tab of the View Configuration panel.