your question is more than unclear.

if a plug-in has a tail or not is usually reflected in it, since VST 1.0 plug-in do tell that to the host.

however, to turn that on or off in practice - in max/msp - is something you will always have to implement yourself (for example by settgin the volume to 0. when the uer presses "stop" on the transport.)

setting volume to 0. is the solution. thank you! before I just turned bypass on/off...

when you need to turn it on again very soon, a plug-in might be still outputting its tail.

here you can add a change of the reverb time parameter to your on/off mechanics. you know, like with tapin~ in MSP: when you set the buffer size to zero, and then to 1000 again, the buffer content will be cleared ...

eventually turning off the plug-in by disable processign of the vst~ object wikl also stop the tail, but that depends on the plug-in.

-110