If you want to hold off on the drip campaigns, you can go to Communication, select Campaigns, then using the toggle next to the word Active, toggle the campaigns to Inactive.
Alternatively, if you're importing leads from a spreadsheet, you can set their drip campaign status to inactive, so they cannot receive any emails until you update the status through selecting all relevant leads, selecting the bulk actions menu, then choosing Drip and Standalone Campaign Status, and setting them to active.