Air Traffic Control Developmental
Confusion; Situational Awareness
Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Provided Assistance
Human Factors; Procedure
Aircraft X called for VFR clearance to practice instrument approaches to SRQ and requested taxi from the FBO.  I read his clearance to him with a proper read back and verified his location for taxi.  After confirming he was; I gave taxi instructions to runway 32 via Alpha; hold short of runway 04.  Aircraft X read back to taxi to runway 32 via Alpha and then made a comment that he could depart from 04; but then changed his mind to taxi to the long runway; runway 32.  I failed to receive the hold short read back.  While taxing on Alpha at a quick speed Aircraft Y was landing on runway 04. As I was observing his taxi I realized Aircraft X was not going to slow down or hold short of runway 04 and immediately instructed him to hold short of runway 04; hold position; and hold now in the same transmission to ensure an immediate stop of his aircraft.  Aircraft X's front half of the aircraft had crossed the hold short line when he came to a complete stop.  Aircraft Y was already on the runway and slammed on his brakes to avoid a collision.  No damage or loss of life ensued.  Aircraft Y was able to taxi around Aircraft X and safely continue to the FBO.I will ensure that I receive hold short instruction read backs.I think all pilots should have a preferred runway and taxi route pre-determined before taxing. This will avoid being distracted during taxi instructions and read back; while they decided which runway they would prefer.
SRQ Tower Controller reports of a runway incursion due to a missed readback.