You need it to power off, if you can't easily open it (to remove a battery) without voiding any warranty, then you would need to wait for it to run out of power and turn itself off. Then, once you have it drained, plug it in to charge but do not turn it on. Let it charge only partially and then turn it on in "Safe Mode". This should allow it to turn on and not load the download(s) so you can uninstall/remove whatever you need to. Then once that is done, restart the device. It should be clear of problems then. Now if that doesn't do it, you would need to do a "Hard Reset" but I would really save that for a last resort option as it will wipe all data on the tablet.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.