Did they just read the error code? If so, it could still be the canister or any of maybe 4 or 5 valves or a pressure sensor in that system. Or a leaky vacuum hose.

We just had error 1457 in the '98 Accord & there's a whole troubleshooting process to check each part of that system. Ours turned out to be a $36 valve attached to the canister. The EVAP canister is underneath the car just forward of the gas tank. Roughly under the LR seat cushion. That's an I-4, I don't know how different the V-6 would be.

What's that about "constant problem"? Have they just been throwing parts at it without actually testing the parts?