There's plenty of different way, each with its pros and cons. It kinda depends on why you want to exclude those cases and what else you'll do with your data.
But anyway, the most likely solution is to compute some filter variable and FILTER on it. For FILTER, see http://www.spss-tutorials.com/spss-filter-command/
The filter variable should have zeros for small categories and ones for large categories so an easy way to create it is to RECODE to original variable INTO a filter variable: http://www.spss-tutorials.com/spss-recode-command/
Without further knowledge, this is the most likely option for your situation.