What must ATC do, following a reported laser sighting by a plane?
Here's what the regulations say ATC should do:

Broadcast on all appropriate frequencies a general caution warning every 5 minutes for 20 minutes following the last report;

Include information on the ATIS broadcast for one hour following the last report;

Report the incident through the Domestic Events Network (DEN), or to their overlying TRACON or ARTCC facility.


As for the pilots, lasers are regarded as an emergency, so you are permitted to take evasive action.
Other aircraft are also permitted to request clearance to avoid the area.

Here are the relevant FAA orders:
FAAO JO 7110.65:

10-2-14. UNAUTHORIZED LASER ILLUMINATION OF AIRCRAFT
a. When a laser event is reported to an air traffic facility, broadcast on all appropriate frequencies a general caution warning every five minutes for 20 minutes following the last report.
PHRASEOLOGY- UNAUTHORIZED LASER ILLUMINATION EVENT, (location), (altitude).
b. Terminal facilities must include reported unauthorized laser illumination events on the ATIS broadcast for one hour following the last report. Include the time, location, altitude, color, and direction of the laser as reported by the pilot.
NOTE- All personnel can expect aircrews to regard lasers as an inflight emergency and may take evasive action to avoid laser illumination. Additionally, other aircraft may request clearance to avoid the area.

FAAO JO 7210.3:

2-1-27. REPORTING UNAUTHORIZED LASER ILLUMINATION OF AIRCRAFT
All FAA Air Traffic Control facilities, Federal Contract Towers and Flight Service Stations shall report unauthorized laser illumination incidents through the Domestic Events Network (DEN), providing the following information:
a. UTC date and time of event.
b. Call Sign, or aircraft registration number.
c. Type of aircraft.
d. Nearest major city.
e. Altitude.
f. Location of event (e.g., latitude/longitude and/or Fixed Radial Distance (FRD)).
g. Brief description of the event.
h. Any other pertinent information.
NOTE- Facilities without direct access to the DEN shall forward the information through the overlying TRACON or ARTCC facility