For services that are defined as actuals (i.e. time and materials), you can define a fixed cap which will then determine the maximum price that can be charged to the client, regardless of the hours put in.
It works like this:
define the service as actuals and define the price and the quantity
click on the lock icon to set a fixed cap:
In this case, the amount that can be billed to the client is set to $75 x 5 hrs, which equals to $375.
If you exceed that amount, the amount that can be billed won't change: