Remove Outliers - Market Basket Analysis
Since I cannot comment to ask for clarification, I am asking it here. What is your reason to think about removing the most frequent value in your dataset? If the second column actually represent frequency of usage, you probably should not remove it and I think it would be illogical to throw away that piece of information. Having said that, you may consider removing that tag if it is a "less meaningful" word (e.g. a, an etc). 

Can you give a little bit more context on what you are trying to achieve?

In general, one way to find outlier is to look at points that lie beyond 1.5 times of inter-quartile range of the distribution i.e. for the frequency count in your data.  

Just a quick thought, did you try clustering for finding similar tags? What are the ways you are considering to find similar tags?