If you have your settings for Immediate Payment required AND have your setting for "don't allow buyers to pay me along with other seller's purchases" (or something to that effect), buyers cannot use eChecks at all to pay for your purchases. I changed my setting to both about 2 1/2 years ago and no longer have to worry about buyers and their NSFs or whinging about why I'm not shipping before it clears.
(For those who don't want to alter their settings or can't because they run auctions, my wording for eChecks used to be, "Thank you for your order. We received your eCheck Payment through PayPal on (date). PayPal requires that we wait for the eCheck to clear before we ship, and they currently estimate that your eCheck will clear (date or range of dates, PayPal gives you these in the eCheck email)."
It's not a lie, either - if I want to be covered by PayPal seller protection, Paypal DOES require us to hold the checks. Just make it sound like very standard policy that you apply and/or PayPal applies to everybody, not directed at the buyer's level of feedback, weird user name, bad breath, etc.