To always display coordinates as DMS values, add this to your common.css:

.geo-default { display: inline }
.geo-nondefault { display: inline }
.geo-dec { display: none }
.geo-dms { display: inline }

To always display coordinates as decimal values, add this to your common.css:

.geo-default { display: inline }
.geo-nondefault { display: inline }
.geo-dec { display: inline }
.geo-dms { display: none }

To display coordinates in both formats, add this to your common.css:

.geo-default { display: inline }
.geo-nondefault { display: inline }
.geo-dec { display: inline }
.geo-dms { display: inline }
.geo-multi-punct { display: inline }

If CSS is disabled, or you have an old copy of MediaWiki:Common.css cached, you will see both formats. (You can either clear your cache or manually refresh this URL: [1].)

To disable display of the blue globe adjacent to coordinates, add this to your common.js:

var wma_settings = {enabled:false}

Note that this will disable WikiMiniAtlas.

See also: Wikipedia:Manual of Style/Dates and numbers § Geographical coordinates.