The 2nd-level Sor/Wiz spell cloud of knives [conj] (Player's Handbook II 107) mentions nothing in its description about the caster being able to suppress its effect voluntarily. It even lacks a bracketed D after its duration, so it isn't dismissible. Thus ending the spell early—even if the spell's generated by a magic item—typically requires dispelling the spell with a dispel magic effect. The spell typically can't be voluntarily suppressed unless, for example, there's a nearby area of antimagic for the subject to walk into. Usually just waiting for the spell's 1 round/level duration to expire is easiest.
On the other hand, an original continuous magic item that incorporates the spell cloud of knives—to my imagination, anyway—would create a cloud of knives effect around the item's wearer or bearer as long as the item's worn or carried. Thus it seems to me that the easiest way to end the effect would be to simply remove or drop the item. For a ring, this DM would likely rule that this takes a move action, and, when complete, the ring is in (not on) the former wearer's hand.
If the actual effect that's desired is more complicated—along the lines of "I never want to remove my magic items; to do so will lead to my death, imprisonment, or worse"—, consider submitting to the DM the following custom magic item:
ring of cloud of knives: This gold ring causes the wearer to be surrounded continually by the effect of a cloud of knives spell. The wearer can take a purely mental free action either to decativate or to restart this effect.
(Ignoring the second sentence, this description uses the language similar to the ring of freedom of movement.) Were a player to submit this original magic item to this DM, I'd compute the market price according to Table 7–33: Estimating Magic Item Gold Piece Values (Dungeon Master's Guide 285). Then, in light of that second sentence, I'd adjust the custom item's base market price from its computed price of 48,000 gp (2,000 gp base ×2 spell level ×3 minimum caster level ×4 for a spell with a duration measured in rounds) upward by 2,000 to 7,000 gp depending on the campaign. (I'd argue that the ability to voluntarily deactivate the effect mentally is more valuable among the rich and powerful in fantasy ersatz Renaissance Italy than it would be on the front lines of the Heaven–Hell War in an ultra-high fantasy setting, for example.)
However, there is an alternative. Consider submitting to the DM the ring of blinking (230) (27,000 gp; 0 lbs.) but change the name (and spells, obviously) to a ring of extended cloud of knives. Yes, the wearer will have to reactivate the item every 14 rounds, but it remains a command word-activated item with a 3rd-level spell effect. (Okay, technically a 2nd-level spell effect modified to 3rd-level, but still.). The two items become approximately equivalent, making comparing them easier on the DM. And to "suppress" the cloud of knives effect created by activating the ring of extended cloud of knives the wearer need only wait at most less than a 90 second for the effect's duration to expire. (To be clear, magic items can be created to incorporate metamagic effects—see here ("Using metamagic feats, a caster can place spells in items at a higher level than normal" (282)) and this Pathfinder answer here that's the same for 3.5.)
