There is no official way to fix the "floating paintbrush bug". It is a bug that is well known, and the focus of many other in-game exploits. There has not been an official patch to fix the bug, and it was not an intended mechanic, so naturally the developers have not included any cheat to "turn the physics back on".
Basically, all paint brushes you drop have a high chance of hovering in the air. These brushes still detect collision, and can even be used as makeshift platforms. In some cases, I have seen this mechanic used as a fix for other, less desirable bugs and behaviors.
The bug has been fixed with the Unofficial Oblivion Patch, which can be applied to the computer platform. Alternatively, if you are playing on console, this bug is reported to be fixed in the PlayStation 3 version of the Game of the Year edition. There is no reported fix on Xbox 360.
Ignoring the Problem:
An alternate way to "fix the problem" is to simply not pick up paintbrushes. Oblivion uses an item weight and capacity mechanic, to prevent your character from moving or fast-travelling when they become 'over-encumbered'. As such, it is counter productive to pick up 'junk'. As paintbrushes do not carry a high value, and serve more to weigh the player down; it would be a good strategy to simply not pick them up, to begin with. From what I understand, this bug can sometimes effect other items, but favors junk.
The best strategy for a PC gamer not wishing to install unofficial modification, a PlayStation gamer stuck with an older copy, or a 360 gamer would be to simply not pick up 'junk items' to begin with. The bug will not effect your experience in any way, and your adventuring will be a tiny bit more efficient for not having to stop to drop loot as often.