Hi V.!
Hang in there! 24 to 36 months can be a really tough age!
Developmentally, each year sort of has a focus. For example, the first year (0 to 12 months) is all about mobility. Reaching, rolling over, crawling, and walking.
The second year (12 to 24 months) is about language aquisition. Not only do they take pride in understanding you, but being understood is hugely motivating!
The third year of a child's life (24 to 36 months) is about cognitive development. They've mastered moving, they feel pretty good about talking and understanding you... they are developing a longer memory and an individual identity. They begin to plan and to have purpose.
About this time, the world as they know it changes too. They have gotten positive attention for standing and climbing and jumping.... we have coaxed them and coddled them and made everyone in a 5 mile radius listen to the cute way they say "turtle". Now, we start expecting that they will use their skills "at appropriate times." They do not understand why we tell them they "can't do that", when they know that they can! They are very literal, have low frustration tolerances, and no delayed gratification skills or impulse control!
Telling him he has 10 minutes until pick up time is a good way to get him ready for transitions, but it doesn't really mean too much to him except that you want him to do it and it means he can't do what he wants to do.
Getting a spanking or a time out at this point is counterproductive to what you want at this point, because he doesn't really care about making you happy. He is initating a power struggle, and everytime it comes down to time out or spanking, he wins.
Use the things that he gets excited about to motivate him. Give him a five minute warning, then a three minute warning, then a one minute warning. If his bath is that exciting for him, take him in the bathroom and let him watch you turn on the tub. Have a "race" to see who can clean up the toys and get in the tub first! Have him pick a few toys to go in the tub with him and put the other toys to bed. Make this a positive transition that he has control over.... get a timer he can set! instead of a power struggle that he wins and you can't tolerate.
Good luck to you and let us know how it goes!