You may have a virus, malware, or a bad app that is doing the installing. You would have to check the tablet for these and remove whatever is the cause.
Should that not work, or you can't figure out the problem, then you may well need to do a full "Hard Reset" of the device to wipe all data and restore it to its original status.
NOTE: Doing this will clear 'all' the data on your tablet, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the tablet...
1. Make sure that the tablet is actually powered off.
2. Remove your SD card, if there is one...