"Stop a loophole" vs. "fix a loophole"

Question

Which is the preferred usage — "to stop a loophole" or "to fix a loophole"?

Answer 1

Neither. The verb you are looking for is close.

Just to demonstrate that that's what people actually use, here are the stats from the Corpus of Contemporary American English (COCA) and the British National Corpus (BNC):

                      COCA    BNC

[close] a loophole     23      3
[plug] a loophole       4      1
[stop] a loophole       0      0
[fix] a loophole        0      0

And for the sake of completeness, here's a list of the top 25 collocations from both corpora for [v*] a loophole:

             COCA                          BNC

 1    FOUND A LOOPHOLE     23     BE A LOOPHOLE         3
 2    IS A LOOPHOLE        16     IS A LOOPHOLE         3
 3    CLOSE A LOOPHOLE     15     FIND A LOOPHOLE       3
 4    'S A LOOPHOLE        11     EXPLOITING A LOOPHOLE 2 
 5    WAS A LOOPHOLE        9     HAVE A LOOPHOLE       2
 6    FIND A LOOPHOLE       9     CLOSE A LOOPHOLE      2
 7    CLOSING A LOOPHOLE    5     SAYS A LOOPHOLE       2
 8    CREATED A LOOPHOLE    4     USING A LOOPHOLE      1
 9    EXPLOITED A LOOPHOLE  4     SAY A LOOPHOLE        1
10    PLUG A LOOPHOLE       4     PLUG A LOOPHOLE       1
11    USED A LOOPHOLE       3     LEAVES A LOOPHOLE     1
12    LEFT A LOOPHOLE       3     CLAIMS A LOOPHOLE     1
13    EXPLOITING A LOOPHOLE 3     BECOMING A LOOPHOLE   1
14    FINDING A LOOPHOLE    3     CREATE A LOOPHOLE     1
15    CLOSES A LOOPHOLE     3     CLOSING A LOOPHOLE    1
16    DISCOVERED A LOOPHOLE 3   
17    CREATING A LOOPHOLE   2   
18    CONTAIN A LOOPHOLE    2   
19    ADDRESS A LOOPHOLE    2   
20    CREATE A LOOPHOLE     2   
21    HAVE A LOOPHOLE       2   
22    HAS A LOOPHOLE        2   
23    OPEN A LOOPHOLE       2   
24    USE A LOOPHOLE        2   
25    SUPPORTED A LOOPHOLE  2   
26    USING A LOOPHOLE      2   
27    LEAVE A LOOPHOLE      2   

As you can see, out of all the various things that various people like to do to various loopholes in various situations, only close and plug fit the bill in yours.

Answer 2

Reverso suggests "close a loophole":

→ The government plans to close some 70 tax loopholes used by companies.
to close a loophole combler une lacune
→ It's time we closed the loophole which allows criminals to gain easy access to lethal weapons

However, plug, fix and stop are also found, in that order.

Google results:

• "close a loophole" — about 1,080,000
• "plug a loophole" — about 230,000
• "fix a loophole" — about 153,000
• "stop a loophole" — about 9,540

Being an idiomatic expression, the choice of the word depends much on the context as well.