How to reduce options for units of measurement to just miles and km for proximity search
Another approach is simply using CSS.

The select list is built like this:

&lt;select&gt;
  &lt;option value="6371"&gt;Kilometres&lt;/option&gt;
  &lt;option value="6371000"&gt;Metres&lt;/option&gt;
  &lt;option value="3959"&gt;Miles&lt;/option&gt;
  &lt;option value="6975175"&gt;Yards&lt;/option&gt;
  &lt;option value="20925525"&gt;Feet&lt;/option&gt;
  &lt;option value="3444"&gt;Nautical Miles&lt;/option&gt;
&lt;/select&gt;


You could hide the undesired values with the following style (in this case only "kilometres" and "miles" remain):

option[value="6371000"], 
option[value="6975175"],
option[value="20925525"],
option[value="3444"]{
  display: none;
}


If you're only using one value, it may look a bit silly to have a select box of it. You could disguise it as regular text though. Make sure the font is identical to the rest, remove the border, set a width to the container of the select list and hide everything seeping out of it, whilst making the select list a tad longer (thus the dropdown arrow gets hidden by the overflow):

.form-item-field-geofield-distance-unit{
    width: 90px;
    overflow: hidden;
}

select{
    width: 120px;
    border: 0;
    cursor: text;
}


Alternatively (Bootstrap 3):

.form-item-field-geofield-distance-unit select{
    border: 0 !important;
    box-shadow: none;
    cursor: text;
    -moz-appearance: none;
    -webkit-appearance: none;
    appearance: none;
}


Adjust the margins or paddings if necessary…