@user3715778's answer is correct. Throwing will not make enough of a difference in the orbit to reenter Earth's atmosphere de-orbit promptly. Let's run the math using the vis-viva equation:
$$v^2 = GM \left(\frac{2}{r} - \frac{1}{a}\right)$$
periapsis apoapsis semi-major periapsis
altitude (km) altitude (km) axis (km) velocity (m/s)
initial 400. 400. 6778. 7668
final 80. 400. 6618. 7575
difference 93
world's record 45
edit: Scott Manley gets about 90 m/s as well.
According to this Quora answer the fastest thrown ball was in Cricket with a speed of about 161 kph or about 45 m/s, which is about half of the speed necessary to promptly de-orbit. This would definitely shorten the time to reentry but it would be quite a challenge while wearing a space suit!
Order hundred meters per second is certainly achievable with a simple slingshot, if sufficiently large and the item sufficiently small. Just a random example: Monster Slingshot New World Record Shot in YouTube, from Joerg Sprave's Slingshot Channel. You might be able to do this in a standard space suit.
A simple sling conceivably might also do it, depending on several details...
However, At 400 km altitude everything could be considered as deorbiting! Drag limits the lifetime of many spacecraft there, and the ISS regularly boosts itself to even maintain it's altitude. The rate is quite variable and activity of the Sun can heat the upper atmosphere and increase decay rate a lot.
For example, Scott Manley says that a tool bag lost from the ISS took only 9 months to reenter the atmosphere for example.
See all the good answers to How long does trash jettisoned by hand from the ISS fall before burning up on reentry? for example. Basically, if you keep something on the ISS it will not de-orbit, at least until the ISS's end of life. However, if you toss it "overboard" it will find it's way to the Earth fairly quickly, and so this is done. In fact, it is done both intentionally and unintentionally, as is explored in the various answers to What kinds of things have been tossed out of the ISS?
GIF:
above: from the question Was this large pieces of “space junk” just released from the ISS in the “nadir and retrograde” direction?
edit: According to Scott Manley's video this is 1998-067NM 43203 ISS DEB
According to Space.com's 02-Feb-2018 article Cosmonauts Break Russian Spacewalk Record During Space Station Antenna Repair:
The cosmonauts spent the day replacing an electronics box for a high-gain communications antenna outside the Zvezda service module. Instead of holding on to the outdated piece of equipment, the cosmonauts tossed the original electronics box overboard, dooming it to burn up in Earth's atmosphere.
This answer shows a plot for a generic satellite, though each size and shape will be different. At 400 km the reentry time ranges from a few years to a few months depending on spacecraft shape and orientation (see this answer for example) and solar activity. More about drag in this asnwer and links within.
This answer links to a site called lizzard-tail.com where you can try a calculation yourself. It won't be accurate, again because spacecraft shape and solar activity can vary, but you can play with some general spacecraft and solar parameters there to get an idea. The website also links to this documentation
above: borrowed from this answer.
above: example of ISS altitude vs time, borrowed from this answer.