Recently a mountain bike --someone's hand-me-down--, came in my way. Size wise it might be slightly big for my ideal but within the range of manageable, I accessed. After trying a few times, however, it became clear that its seat post becoming too long and saddle too high with the way it was. I broke the parts apart and the only way I could make do with this seemed not to use this post or any post at all, crazy it may sound, but it should work out only if the saddle cushion part immediately placed on the seating area. (Somehow the depth that the post can slide down is limited to half of the post length due to its structure: a "coil type' object obstructing and blocking the post. I'm a girl of 5 foot 2. Although the bike is a mountain type, I am not planning to use this for adventurous rides in mountains or slopes but for small scaled travel nearby with no bumps and/or environments as parks, etc. I am an experienced cyclist but also a city dweller, who would be cautious due to frequent casualties observed in the area where I am currently located: urban.
I think what you are saying is that you need to drop the seat post all the way down into the frame, so the saddle is as close to the frame as possible. You need a seat post to connect the saddle and frame. You can't attach the saddle directly to the frame.
If the post cannot be slid all the way into the frame due to an obstruction it can be cut down to shorten it. As long as the minumum length is inserted into the frame you will be structurally safe. You can buy a shorter post of course.
However, if you need to drop the saddle all the way to the frame the bike is likely too large for you and possibly dangerous, as you will have no standover clearance.