Can you cast a kicked spell for its base cost without paying the kicker?

Kicker is an optional additional cost defined in CR 702.33. When you cast a spell that has kicker, you choose whether or not to pay the kicker cost as part of announcing the spell and determining its total cost. There is no requirement to pay it.

If you choose not to pay the kicker, the spell is simply cast for its base mana cost and none of the kicker's bonus effects apply. The spell is not considered 'kicked,' so any ability that says 'if this spell was kicked' will not trigger or apply.

This is true even if paying the kicker would be advantageous — the choice is entirely yours each time you cast the spell. Some spells have multiple kicker costs (sometimes called 'multikicker'), and you may pay each of those zero or more times independently as well (CR 702.33c).

Example: You cast Kor Sanctifiers (base cost 2W, kicker 1W). You only have 3 mana available. You pay 2W for the base cost, choose not to pay the kicker, and Kor Sanctifiers enters the battlefield as a 2/3 Cleric — but since it was not kicked, its 'if this spell was kicked, destroy target artifact or enchantment' ability does not trigger.

Relevant rules: CR 702.33a establishes kicker as an optional additional cost, and CR 601.2b covers choosing to pay or not pay optional additional costs during spell announcement.