text .t
pack .t -fill both -expand 1

bind blocker <BackSpace> {bs}
bindtags .t [concat blocker [bindtags .t]]

proc bs {} {            
 bind blocker <BackSpace> {puts "Eating it.";break}
 puts "Handling it."                  
# simulate a workload.
 for {set i 0} {$i < 20} {incr i} {puts "$i;[update]"}
 bind blocker <BackSpace> {bs}
}

.t insert 0.0 "asfasdljkas;lkj;lsf;lkjasdfl;jf"