you might need to use a set up that involves using two (or more) separate delay lines and swapping back and forth between them at the moment you want to 'clean' the delay. here's how it would work:

assume two delwrite~ obects with delay lines named a and b. matching these two delwrite~s are two delread~s. each of these four objects can be 'turned on' or 'muted' (using the method supplied above by giair). muting a delwrite~ object technically means muting the audio going into to the object.

the initial state of the patch would [delwrite~ a] and [delread~ a] turned on and [delwrite~ b] and [delread~ b] muted. when you want to clear the delay line, you would mute [delwrite~ a] and [delread~ a]. at that point you can turn on the 'b' delay line objects. you can switch back and forth between your two delay lines cleanly if you wait an amount of time greater than the length of the delay.

in the case that your delay time is longer than the time you want to wait, you would have to use another delay line. the downside of this is that the control gets a little bit more complicated. if this is something you might apply, check miller puckette's book (pdf available on his website i think) and look at the chapter on polyphonic synthesizers. polyphonic synths require some form of voice allocation and you might be able to use some of pd's voice allocation support to help manage your delay lines, because it is a fairly similar process.

i hope this made sense. i don't think it's as confusing as it sounds, but it is hard to put into words. if you understand what i mean, but aren't sure how to implement it, i could probably post an example; just let me know.

--zac