You MUST align the slide/frame, as illustrated in BMCM's post/pic, to create clearance for the raised portion of the slide stop lever when pressing it out the left side of the frame.
In this pic notice the raised steel ledge on the top surface of the lever body inside the yellow ring. This raised steel area of the slide stop's body can
only clear the inside of the slide if the slide is properly positioned.
I've had any number of guys & gals bring me their 3rd gen's at a cleaning station at our range, telling me their slide stops are "stuck" and they can't field-strip their pistols.
Granted, occasionally you may find a slide stop body (curved notch in pin) and/or guide rod plunder (head) that may have developed a noticeably burred spot on one or the other, and it makes moving the slide stop pin across the compressed guide rod plunger a bit more difficult. Normal machining marks on the pin bodies, inside the curved cut, are ... normal. I sometimes add a very slight dab or oil or grease at that spot.
However, in my experience the significant number of instances where someone was having trouble field-stripping their 3rd gen gun wasn't due to burrs, but to an improperly positioned slide notch not allowing clearance for the slide stop's raised ledge to clear the slide rail.
Even when I demonstrated how to hold the slide/frame in the proper relationship (using the same 1-handed grip shown by BMCM), and then had the issued user try it, very often the person would start with the right grip on the slide/frame (which is under tension from the recoil spring, remember) ... but then forget to keep the exact slide/frame alignment while transferring his/her attention to the other side of the gun, pushing on the right end of the slide stop pin ... and the slide/frame alignment would shift just enough to
again put the slide in the way of the raised ledge of the slide stop body.
In other words, they were trying to shove steel through steel, and the steel-against-steel contact was always going to defeat their efforts. It doesn't take much "slippage" of the slide's position to lose the proper notch alignment over the top of the slide stop, and I sometimes had to repeatedly demonstrate how they had to keep the slide/frame alignment in the proper relationship while shifting their attention over to the other side of the gun and pushing against the other end of the slide stop body's pin. It's easy to lose the attention needed for one part of the task when shifting focus on another part of the task, especially when a recoil spring is constantly trying to push the slide forward.
Now, if the slide stop's raised ledge (or a part of the slide's rail on either side of the takedown notch) has been damaged by someone having beaten on or otherwise tried to force the steel ledge
through the steel slide rail? Well, that's another sort of problem.
While I've never personally encountered a damaged guide rod plunger spring that wouldn't permit the plunger to be depressed, that's also something I check, looking for normal movement of the plunger in the old style "pinned/staked" guide rods. In the newer revision of the guide rod assemblies the plunger spring and plunger can be removed and replaced (spring snaps onto the plunger bases).
Experiencing difficulty in field-stripping a 3rd gen
isn't a matter of "using a hammer" to beat on the right end of the assembly pin, and especially
not a bigger hammer 
, but aligning (and holding) the slide over the frame to create the necessary clearance so the slide stop can be moved out the left side of the frame & slide.
Sure, every once in a while, in a very old and very dirty gun, I might need to use the flat end of a handy plastic stick pen to push against the right end of the pin (hand pressure only) to get it moving.
Just some thoughts.