2 added 1763 characters in body
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                       COCA    BNC

[close] a loophole       23      3
[stop][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:

     0        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.

                       COCA   BNC

[close] a loophole       23     3
[stop] a loophole         0     0
[fix] a loophole          0     0
                      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.

1
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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
[stop] a loophole         0     0
[fix] a loophole          0     0