Remember, cats understand territory, and can be discouraged/"trained" to stay away. Now that spring is here, I have to reassert my claim to the winter-neglected garden.
Do you have anything planted there right now?
Since they like to scratch around, one solution might be to use chunky bark mulch if all you have are shrubs.
For areas where I want to grow flowers, or even edibles, I have taken to using extreme measures. One is to put rose and thorny bramble trimmings around the bed. (These are temporary and the intent is to discourage and break them of the habit of making this a regular stop.) Another is to sprinkle ground black peppers. When the offender continues, I move on to ground hot peppers. They always sniff the area before using it.
You need to scrub down any part of the porch or house that may have been sprayed by male cats. It's a good idea to use organic enzyme cleaner sold for the purpose, but avoid ones that have fragrance in them unless you want your front porch to smell like perfume.
In my case, my own cats are indoors only with supervised outside play time in the fenced yard. So they are all neighbors' or stray cats. I've already warned my neighbors that I will be taking these measures to protect my garden beds.
(when I complained, some of their responses we're "well, they're cats. We can't control where they go" but my feeling is that if dog owners are required by law to pick up after them, cat owners should have the same responsibility.) I don't enjoy encountering cat poop in my garden.
If you have a cat yourself, the smell in the basement maybe because your cat is peeing/has peed there somewhere. One of my cats started peeing in the garage and it took me a long time to track down the smell.