Why not use something like a keypress delay value eg.
SETSCREEN 640,480,1
LOCAL quit%=0
LOCAL myvar$
WHILE quit=0
PRINT "PRESS ENTER",20,20
IF KEY(28) THEN quit=1
SHOWSCREEN
WEND
LOCAL press%=20
WHILE myvar$=""
IF press=0 THEN INPUT myvar$,0,0,TRUE
IF press>0 THEN DEC press
SHOWSCREEN
WEND
What this does is ignore any key presses until the "press" value has decreased to zero (once per frame - so in this case 20 frames).