Don't do that then.
Most likely you are getting the 1-2 upshift when you let off the gas, and actually this is fairly normal and the only thing you can do about it is either back off the TP or increase it.
Stock the trans is adjusted with less TP than yours is now so they don't do that as noticeably, but if you want slightly later firmer shifts you have to put up with it.
The basics of what's happening is that the trans is delaying the 1-2 because you have the throttle on a bit, but when you let of the gas it is suddenly not under that load any longer and so it shifts.
Now if you raise the TP farther it will firm up as well as delay that 1-2 shift even longer and not make the shift in a parking lot at all.
It also gives you engine braking around parking lots because it stays in first.
Now that you know all that I would have to recommend you back off on the TP a little and make it shift a little sooner and that bang into second will pretty much be gone.
It will also make your shifts earlier and softer.
It will always be there in certain circumstances, but it'll just be a lot softer and less noticeable, so it's a trade off.
I adjust mine maybe .030 at a time and then drive it like I normally do for a bit before I make another change if it's needed.
