Unfortunately, "tightening" the focus mechanism isn't as simple as tightening some set screw.

• Your lens may need to be cleaned, lubed, and adjusted (CLA). If it were an ancient manual-focus lens that you didn't mind breaking, you could try to clean it yourself. I'd probably even try disassembling an old FD-mount L lens. However, it's best to refrain from messing with the internals of any lens with electronics, unless it's already broken.

  Since your lens is fairly new, CLA is unlikely to be helpful. You can go to a camera store to see if other copies of the lens behave the same way.

• As a workaround, you can try partially covering the focus ring with a wide rubber wrist band to add a little bit of resistance. How successful this is likely to be depends on the underlying cause of the focus changes. For instance, one of my lenses had too much free play in the helical groove. I fixed it by making a shim to prevent unwanted movement. Adding resistance to the focus ring would have done nothing to prevent focus shifts.