diff -up dwm-6.2.orig/config.def.h dwm-6.2/config.def.h
--- dwm-6.2.orig/config.def.h	2021-04-17 17:58:27.803010601 +0200
+++ dwm-6.2/config.def.h	2021-04-19 09:04:26.224804870 +0200
@@ -26,9 +26,9 @@ static const Rule rules[] = {
 	 *	WM_CLASS(STRING) = instance, class
 	 *	WM_NAME(STRING) = title
 	 */
-	/* class      instance    title       tags mask     isfloating   monitor */
-	{ "Gimp",     NULL,       NULL,       0,            1,           -1 },
-	{ "Firefox",  NULL,       NULL,       1 << 8,       0,           -1 },
+	/* class      instance    title       tags mask     isfloating   monitor     notallowed */
+	{ "Gimp",     NULL,       NULL,       0,            1,           -1,         0 },
+	{ "Firefox",  NULL,       NULL,       1 << 8,       0,           -1,         0 },
 };
 
 /* layout(s) */
diff -up dwm-6.2.orig/dwm.c dwm-6.2/dwm.c
--- dwm-6.2.orig/dwm.c	2021-04-17 17:58:27.803010601 +0200
+++ dwm-6.2/dwm.c	2021-04-19 09:03:42.571206643 +0200
@@ -92,7 +92,7 @@ struct Client {
 	int basew, baseh, incw, inch, maxw, maxh, minw, minh;
 	int bw, oldbw;
 	unsigned int tags;
-	int isfixed, isfloating, isurgent, neverfocus, oldstate, isfullscreen;
+	int isfixed, isfloating, isurgent, neverfocus, oldstate, isfullscreen, notallowed;
 	Client *next;
 	Client *snext;
 	Monitor *mon;
@@ -139,6 +139,7 @@ typedef struct {
 	unsigned int tags;
 	int isfloating;
 	int monitor;
+	int notallowed;
 } Rule;
 
 /* function declarations */
@@ -297,6 +298,10 @@ applyrules(Client *c)
 		&& (!r->class || strstr(class, r->class))
 		&& (!r->instance || strstr(instance, r->instance)))
 		{
+			if (r->notallowed) {
+				c->notallowed = r->notallowed;
+				break;
+			}
 			c->isfloating = r->isfloating;
 			c->tags |= r->tags;
 			for (m = mons; m && m->num != r->monitor; m = m->next);
@@ -1029,6 +1034,7 @@ manage(Window w, XWindowAttributes *wa)
 	c->w = c->oldw = wa->width;
 	c->h = c->oldh = wa->height;
 	c->oldbw = wa->border_width;
+	c->notallowed = 0;
 
 	updatetitle(c);
 	if (XGetTransientForHint(dpy, w, &trans) && (t = wintoclient(trans))) {
@@ -1037,6 +1043,21 @@ manage(Window w, XWindowAttributes *wa)
 	} else {
 		c->mon = selmon;
 		applyrules(c);
+		if (c->notallowed) {
+
+			XMapWindow(dpy, c->win);
+			if (!sendevent(c, wmatom[WMDelete])) {
+				XGrabServer(dpy);
+				XSetErrorHandler(xerrordummy);
+				XSetCloseDownMode(dpy, DestroyAll);
+				XKillClient(dpy, c->win);
+				XSync(dpy, False);
+				XSetErrorHandler(xerror);
+				XUngrabServer(dpy);
+			}
+
+			return;
+		}
 	}
 
 	if (c->x + WIDTH(c) > c->mon->mx + c->mon->mw)