This additional information I'm providing years later may already be represented elsewhere in the forums, not sure ...

Once, years ago, having the same problem, I discovered that I could place my cursor before the colon and "delete" it right within the text as it appears on the page, however, if you right-clicked and did Update Field, the colon would return.

I found the better solution below, adding additional code at the end of the field, at the webpage Combine Cross-references

Adding an additional backslash (see below) with a hashtag followed by a space and then the number zero will cut off any trailing characters. For example ...

REF _Ref149150908 \r \t \# 0

Curiously, I found this solution years ago and I've just discovered that something has changed, I'm assuming in Word's programming ... entering the additional code works to permanently block additional characters still, however, if you close and then re-open the controls (right-click and Edit Fields), the "\# 0" is gone. It's still blocking additional characters, however, and so it still seems to be a solution, but there doesn't appear (on my system configuration and software versions) to be any trace of that truncation having been applied when viewing the code at a later date.