How to lower the electrical conductivity without diluting?

Generally, a solution is more conductive if it has unpaired ions (that is, elements with a charge of positive or negative).  In order to make the solution less conductive, you would add a dried chemical (it could not be a liquid solution, or that would dilute the original) with the proper charge (an acid if the solution is basic, or vice versa).  This would not affect the concentration of the original element of the solution, but if it is paired with an element of the opposite charge, it would no longer be a good conductor of electricity.