A simple look at any number of pages on tw policy writing (example here) will tell you how to include a stop point in your policy.
Quoted from the above ref.:
Stop Points: Stop points are used to specify specific files or directories that Tripwire should not scan. The syntax for stop points is:
! object_name ;*
EDIT: If, per yr comment below, you only want to check on file modifications to the exclusion of "creation" & "deletion", there may be a problem of semantics in the interpretation of that specific policy rule, for creation and deletion of a file may be logically considered as "modifications". You might want to check on that. If so, the 1st property mask below may give you false positives in the sense that you will detect file modifications even if those flags are raised as the result of a creation or a deletion. Read on.
Meanwhile, see if the following property masks (as explained in the reference given in the 1st line) do it for you:
/fully/qualified/path/to/object -> +m-i;
/fully/qualified/path/to/object -> +M;
Explanation, see the man page:
- m --- Modification timestamp
- i --- inode's number
- M --- MD5 hash value (only checks on file content, not permissions, etc.)
+ means "record and check the following property"
- means "ignore the following property"
I assume that you are already familiar with inodes. If not StartPage is your friend... You will probably need to experiment a bit.