There are two ways which this can be accomplished depending on how/who put the repo into the system
in /etc/apt/sources.list is the generic file where repositories are. You can comment out the line or delete the line(s) which have the repo in question.
The second way is to go into the software manager, click edit -> software sources. Most likely the repo in question will be under "Other Software" tab. Highlight the repo to change and either edit or remove it