I don't have RSLogix handy, so I'm pulling these from memory - some might not work:
Method 1: CLR ST16:0.LEN - just set the string length to zero - the data will go 'poof'
Method 2: CLR N7:0 CLR N7:1 COP #N7:0 ST16:0 2 - the first word in the string is the length - when you copy an integer to a string, the first word goes into the Length, the rest as ASCII.
Method 3: FLL 0 ST16:0 1 - same as 2, just cleaner.
Method 4: Prepare ST16:1 to be "null". COP #ST16:1 #ST16:0 1.
Let me know which one of these works, if any.