ZBW controller described conflict event when visual separation descent clearance was rescinded but air carrier descended and separation was lost.

Narrative

Air Carrier X was cleared to climb to 14;000 FT. Air Carrier Y was descended to 15;000 FT. Air Carrier X was issued traffic 12 O'clock position; 2 miles; same direction; same route at 15;000 FT. Air Carrier Y reported traffic in sight. Air Carrier X was issued 'maintain visual separation with company E-135 who will be descending through your altitude.' Air Carrier X acknowledged. At this point; I decided not to descend Air Carrier Y as I was uncomfortable with the Pilot readback and the proximity of the 2 aircraft. I advised Air Carrier X to 'disregard; I am going to leave Company above you.' Air Carrier X then said; 'I have traffic at my altitude; 15;000 FT about 1 or 2 miles. I think we need a turn to the left.' At this time; Air Carrier X's Mode C indicated level at 14;000 FT; then the next hit was 14;800 FT. I advised Air Carrier X he was assigned 14;000 FT; and turned him to a 90 degree heading (approximately 110 degree turn to left); then turned Air Carrier Y to a 280 degree heading (approximately 90 degree right turn). Air Carrier X stated he read back 15;000 FT; which upon review of FAA audio tapes was incorrect. Air Carrier X was originally issued 14;000 FT and read back 14;000 FT. Neither aircraft reported a TCAS alert or advisory. Closest proximity about 1 mile in-trail and zero vertical.