What is already known about rigid line arrangements? By line arrangement, I mean a unions of lines in $\mathbb{P}^2\_{\mathbb{C}}$ with fixed incidences. (Written in notation, I mean a collection of lines $\mathcal{L}$, a collection of points $\mathcal{P}$, and a set of incidences $\mathcal{I}\subset \mathcal{L}\times\mathcal{P}$ that determine which points lie on which lines.)


By rigid, I mean that all the unions of lines that satisfy the conditions of the arrangement are in the closure of the same $PGL\_3$ orbit. 


For example, we can pick four points $p\_1,\ldots,p\_4\in\mathbb{P}^2$ and draw the six lines between them. This arrangement is rigid because $PGL\_3$ is 4-transitive on points.


For a more interesting example, we can let $p\_1,\ldots,p\_9$ be the nine flexes of a smooth plane cubic and draw the twelve lines that pass through three of the flexes. A proof of rigidity is given in <http://alpha.math.uga.edu/~luca/hesse.pdf>, but the idea is to first note that the flexes don't move in Hesse's pencil, so the construction is independent of j-invariant. Then, given a configuration of 12 lines with the same incidences as the Hesse configuration, you find a smooth cubic passing through the 9 points of concurrency, and then use all the relations you have from the 12 lines to show that those 9 points must be flex points.


Unfortunately, my knowledge ends here. I don't know any other examples except for trivial ones, and I couldn't find anything with an internet search.



Rigid line arrangements
You have to be a bit more careful with your definition of arrangements and their rigidity. Once, you are, then "Mnev universality" provides an enormous supply of rigid arrangements in $P^2$ (even on scheme-theoretic level). See Theorem 1.3 in 
my paper with John Millson ["On representation varieties of Artin groups, projective arrangements and the fundamental groups of smooth complex algebraic varieties"](https://www.math.ucdavis.edu/~kapovich/EPR/ihes.pdf). 


