Enroute Controller described a loss of separation likely caused when an aircraft initiated an unauthorized climb into an occupied altitude; the reporter acknowledging additional precautions might be necessary when controlling general aviation aircraft.

Narrative

I may have transposed the call signs of the aircraft in this report; I only have the call signs right now and I'm not sure which pilot made the deviation. That said; I believe that it was Aircraft X that was at four thousand; requesting higher. I had stopped him at 040 for traffic at 050. I advised the pilot that it would be a while before I could issue his requested altitude of 070 because of said traffic; and inquired whether he would like 040 as a final or prefer a vector to continue his climb. He advised that he would take a vector. I issued a heading of 220 to separate him from the 050 traffic and gave the reason for the vector as 'for your climb.' I did not clear the pilot to leave 040; however he accepted the vector and began to climb immediately. As soon he left 040 the pilot lost minimum separation with the other aircraft. I saw that he was climbing through 044 and told the pilot 'maintain four thousand.' The pilot returned to 040. Recommendation; while the pilot clearly deviated from his altitude clearance; I could have restated his assigned altitude when issuing the vector so as to preclude any ambiguity. Particularly with general aviation pilots; it is sometimes best to take additional precautions to ensure separation.