In general I find it is better to write non 7-bit ASCII strings as HEX literals in the SAS code. That way the code works the same if you change the encoding setting you are running with.
So LOOK at the hex codes for the character by using the $HEX format.
Then write the code using the hexcode.
For example if that "box" is was just the hex code '0A'x (which is actually a linefeed) then you might use
APPLCNT_CMT_TX=compress(APPLCNT_CMT_TX,'0A'x);
If the goal is to only keep normal characters then use the K modifier on the COMPRESS() function call to say that the list of characters are those that should be KEPT instead the default action of removing the listed characters. You can use COLLATE() to generate all of the printable 7-bit ASCII characters.
APPLCNT_CMT_TX=compress(APPLCNT_CMT_TX,collate(32,126),'K');