I would recommend reading about the state system, it's very powerful and flexible:
The State System is an incredibly powerful system built into the Ultimate Character Controller. Any game will have the character switching between component values as . . .
opsive.com
The basics of what you need to do are:
1. Give the Use ability a state name (under the Use item ability there is a "State" field - the demo scene already has the state name "Use")
2. Add a new preset to the Ultimate Character Controller component and set this component up to set the motor speed to 0
3. Set the state name for this preset to "Use" (or whatever state name you gave the Use ability)