prevent indentation of a paragraph

Right click in the paragraph, click Paragraph Settings, uncheck Indent Paragraph in the lower left corner of the dialog box. (This does the same as Werner's comment, \noindent is added to the LaTeX source, but you do not need the ERT.)

In my case, I didn't want the identation for texts coming right after an equation. I was following the approach suggested by Torbjørn T., but it was kinda cumbersome in my case, since I had a lot of equations, and I didn't find any keyboard shortcuts for doing it.

However, I've just noticed that if you continue to write right after the equation (without pressing the Enter key) the line will continue as if it were part of the same paragraph, which was exactly what I was looking for.